Fix FLAG_NOT_FOCUSABLE ime target

Windows with FLAG_NOT_FOCUSABLE can not be IME target unless alpha is
zero (i.e. app is using transparent window to detect IME for legacy
reasons).
Going forward we can improve this with the todo already mentioned in the
code block.

Bug: 145812508
Fix: 149899590
Test: Manually verified using steps in bug.

Change-Id: I0e7db484dd4f9b584998b9f922fd46030fb6fb72