Original PR: #32877

Revert of #32877: commit 1965c92 (#33471)

This patch: Revert of #33471

fxbug.dev/85125

This PR switches the GfxPlatformView code to use the TouchSource/MouseSource API. After this PR, both GFX and Flatland will receive touch and mouse events over the new APIs, instead of the legacy GFX SessionListener API.

flutter/flutter#102412

=Fuchsia testing=
$ fx test //src/ui/tests
(This is what broke the roll last time)

=Flutter testing=
Test re-enabled: PlatformViewTests.TouchSourceLogicalToPhysicalConversion
$ fx shell run fuchsia-pkg://fuchsia.com/flutter_runner_tests#meta/flutter_runner_tests.cmx

=Product testing=
This PR has been manually validated on a nelson device (today, July 29) running stable production code, with the following UI cases:

• ordinary usage of the System UI
• with child view (YouTube), bring up settings (Sys UI)
• with a partial-screen child view (Duo) interrupting another full-screen child view (YT), and both are still responsive to touch
• with and without the new injection2 code path for child views

Pre-launch Checklist

• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• I read the [Tree Hygiene] wiki page, which explains my responsibilities.
• I read and followed the [Flutter Style Guide] and the [C++, Objective-C, Java style guides].
• I listed at least one issue that this PR fixes in the description above.
• I added new tests to check the change I am making or feature I am adding, or Hixie said the PR is test-exempt. See [testing the engine] for instructions on
  writing and running engine tests.
• I updated/added relevant documentation (doc comments with ///).
• I signed the [CLA].
• All existing and new tests are passing.