Track by which client the last focused window was reported.

To diagnose race conditions like issue #25373872, we want to track
which input method client corresponds to IMMS#mCurFocusedWindow.

Note that IMMS#mCurClient does not always correspond to
IMMS#mCurFocusedWindow, because input method clients can report the
window focus gain only, without requesting to be bound to IME.

Bug: 25373872
Change-Id: Iad121669c0f3db4461245dc80ff3fdee469abb79