Add alpha support to Insets API

All insets side may want to animate with transparency with or without
insets. This change allows any side to do that.
Also, this CL adds fade to IME when its not controlled by app.
This tries to replicate original IME animation.

Bug: 111084606
Test: Manually using test app.

Change-Id: I1ecd4715336a58cd904b2898bb36462e5ce2b6ab