Addresses the UI portion of #877.

A separate PR will integrate passing the custom long-press duration into KeymanWeb.

The slider in the menu has discrete positions from [0,7] and correspond to long-press durations 0.3s to 1.5s.
Internally, they code keeps track of the current long-press delay as milliseconds, and the default KeymanWeb duration is 500ms.

Screenshots

The menu to adjust the long-press delay is accessed in Keyman Settings

Position at default 0.5s

Position at 1.3s

User Testing

Setup - Install the PR build of Keyman for Android

• TEST_MENU - Verifies the value of the longpress delay can be adjusted from 0.3s to 1.5s

1. Launch Keyman and dismiss the "Get Started" menu
2. In Keyman, go to Settings --> Adjust longpress delay
3. In the "Adjust longpress delay" menu, verify the default is 0.5 seconds
4. Click the left arrow (by the slider) until the minimum delay is reached
5. Verify the delay time says 0.3 seconds
6. Click the back arrow and exit back to the Keyman app
7. In Keyman, go to Settings --> Adjust longpress delay
8. Verify the delay time says 0.3 seconds
9. Click the right arrow (by the slider) until the maximum delay is reached
10. Verify the delay time says 1.5 seconds
11. Click the back arrow and exit back to the Keyman app
12. In Keyman, go to Settings --> Adjust longpress delay
13. Verify the delay time says 1.5 seconds