This is a follow up to the following pull requests:
- flutter#124514

I was finally able to reproduce this bug and found out why it was happening. Consider this code:

```dart
GestureDetector(
  behavior: HitTestBehavior.translucent,
  // Note: Make sure onTap is not null to ensure events
  // are captured by `GestureDetector`
  onTap: () {},
  child: _shouldShowSlider
    ? Slider(value: _value, onChanged: _handleSlide)
    : const SizedBox.shrink().
)
```

Runtime exception happens when:

1. User taps and holds the Slider (drag callback captured by `GestureDetector`)
2. `_shouldShowSlider` changes to false, Slider disappears and unmounts, and unregisters `_handleSlide`. But the callback is still registered by `GestureDetector`
3. Users moves finger as if Slider were still there
4. Drag callback is invoked, `_SliderState.showValueIndicator` is called
5. Exception - Slider is already disposed

This pull request fixes it by adding a mounted check inside `_SliderState.showValueIndicator` to ensure the Slider is actually mounted at the time of invoking drag event callback. I've added a unit test that will fail without this change.

The error stack trace is:

```
The following assertion was thrown while handling a gesture:
This widget has been unmounted, so the State no longer has a context (and should be considered
defunct).
Consider canceling any active work during "dispose" or using the "mounted" getter to determine if
the State is still active.

When the exception was thrown, this was the stack:
#0      State.context.<anonymous closure> (package:flutter/src/widgets/framework.dart:950:9)
#1      State.context (package:flutter/src/widgets/framework.dart:956:6)
#2      _SliderState.showValueIndicator (package:flutter/src/material/slider.dart:968:18)
#3      _RenderSlider._startInteraction (package:flutter/src/material/slider.dart:1487:12)
#4      _RenderSlider._handleDragStart (package:flutter/src/material/slider.dart:1541:5)
#5      DragGestureRecognizer._checkStart.<anonymous closure> (package:flutter/src/gestures/monodrag.dart:531:53)
#6      GestureRecognizer.invokeCallback (package:flutter/src/gestures/recognizer.dart:275:24)
#7      DragGestureRecognizer._checkStart (package:flutter/src/gestures/monodrag.dart:531:7)
#8      DragGestureRecognizer._checkDrag (package:flutter/src/gestures/monodrag.dart:498:5)
#9      DragGestureRecognizer.acceptGesture (package:flutter/src/gestures/monodrag.dart:431:7)
#10     _CombiningGestureArenaMember.acceptGesture (package:flutter/src/gestures/team.dart:45:14)
#11     GestureArenaManager._resolveInFavorOf (package:flutter/src/gestures/arena.dart:281:12)
#12     GestureArenaManager._resolve (package:flutter/src/gestures/arena.dart:239:9)
#13     GestureArenaEntry.resolve (package:flutter/src/gestures/arena.dart:53:12)
#14     _CombiningGestureArenaMember._resolve (package:flutter/src/gestures/team.dart:85:15)
#15     _CombiningGestureArenaEntry.resolve (package:flutter/src/gestures/team.dart:19:15)
#16     OneSequenceGestureRecognizer.resolve (package:flutter/src/gestures/recognizer.dart:375:13)
#17     DragGestureRecognizer.handleEvent (package:flutter/src/gestures/monodrag.dart:414:13)
#18     PointerRouter._dispatch (package:flutter/src/gestures/pointer_router.dart:98:12)
#19     PointerRouter._dispatchEventToRoutes.<anonymous closure> (package:flutter/src/gestures/pointer_router.dart:143:9)
#20     _LinkedHashMapMixin.forEach (dart:collection-patch/compact_hash.dart:625:13)
#21     PointerRouter._dispatchEventToRoutes (package:flutter/src/gestures/pointer_router.dart:141:18)
#22     PointerRouter.route (package:flutter/src/gestures/pointer_router.dart:127:7)
#23     GestureBinding.handleEvent (package:flutter/src/gestures/binding.dart:488:19)
#24     GestureBinding.dispatchEvent (package:flutter/src/gestures/binding.dart:468:22)
#25     RendererBinding.dispatchEvent (package:flutter/src/rendering/binding.dart:439:11)
#26     GestureBinding._handlePointerEventImmediately (package:flutter/src/gestures/binding.dart:413:7)
#27     GestureBinding.handlePointerEvent (package:flutter/src/gestures/binding.dart:376:5)
#28     GestureBinding._flushPointerEventQueue (package:flutter/src/gestures/binding.dart:323:7)
#29     GestureBinding._handlePointerDataPacket (package:flutter/src/gestures/binding.dart:292:9)
#30     _invoke1 (dart:ui/hooks.dart:186:13)
#31     PlatformDispatcher._dispatchPointerDataPacket (dart:ui/platform_dispatcher.dart:433:7)
#32     _dispatchPointerDataPacket (dart:ui/hooks.dart:119:31)

Handler: "onStart"
Recognizer:
  HorizontalDragGestureRecognizer#a5df2
```

*List which issues are fixed by this PR. You must list at least one issue.*

Internal bug: b/273666179, b/192329942

*If you had to change anything in the [flutter/tests] repo, include a link to the migration guide as per the [breaking change policy].*