Input: Let WM compute touchable region 2/2

- For modal windows let WM define the touchable region. This fixes a regression introduced by
  ag/5700485 where input was sent to background windows.
- When calculating input surface position, ignore the transparent region so that we do not
  incorrectly offset the input. This fixes and issue with input offset in AutoDesk drawing app.

Bug: 120615996, 120612739, 120585467, 120604247
Test: Manual testing with apps listed in bugs. Automated tests to follow

Change-Id: Ie5165a608c3bc1dd9097693b631eb4834e8e681d