When an opaque window has alpha < 0.5, don't let it receive touches.

When a window is declared as opaque but the alpha is low, it's likely a fadeout animation or other visual change that should not affect touch handling.

Bug: 266433089
Test: inputflinger_tests
Flag: EXEMPT bug fix
Change-Id: I9ba48365d0ea080a4e68c8c497049225b759df79