Unify windowGainedFocus() and startInput().

This is a safe refactoring that changes nothing.

In order to improve the keyboard dismissal lags [1][2], we have used
IMMS#windowGainedFocus() as a combined event to do startInput() in
certain situations.

To make the intent of those CLs clear, this CL renames
IMMS#windowGainedFocus() to IMMS#startInputOrWindowGainedFocus().  Note
that these are @hide internal IPC protocols.  Hence this change is never
observable to application developers.

 [1] I8494cbd6e19e2ab6db03f2463d9906680dda058b
     a82ba54b
 [2] Icb58bef75ef4bf9979f3e2ba88cea20db2e2c3fb
     7663d80f

Bug: 25373872
Change-Id: I56934f18e30d90fcdf77bcbb0c35a92a5feb1b82