Input: Let WM compute touchable region 1/2

- 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
Test: Manual testing with apps listed in bugs. Automated tests to follow
Change-Id: I906f1396ccc0c76f602a9903e696eedfc026f7c2
(cherry picked from commit f590f113eee4b24de86bef71c92c1f7fd206e0ac)