Introduce @SoftInputModeFlags annotation

Doing this allows us to annotate integer values that should corresponds
to WindowManager.LayoutParams#softInputMode.

Test: There should be no behavior change.  RetentionPolicy.SOURCE
      annotation should change nothing in production code.
Test: checkbuild
Bug: 32784563
Bug: 35079353
Change-Id: I96300b090edce327d0515c740183abe91ded6bac