Add WINDOW_GAINED_FOCUS to IntDef annotation

This CL adds the missing WINDOW_GAINED_FOCUS annotation to the
StartInputFlags IntDef declaration to avoid compiler warnings such as
the one in IMMS#startInputOrWindowGainedFocusInternalLocked.

Also, this CL replaces decimals with bit-shift expressions as per AOSP
style guide.

Test: make
Bug: 205676419
Change-Id: I51fe293149d6f7fab5790d13764449d62cbdb668