Enable USE_REPORT_WINDOW_GAINED_FOCUS_ASYNC

We had CL[1] to construct an async mechanism if it
only report the focus gain in InputMethodManager.
And using a USE_REPORT_WINDOW_GAINED_FOCUS_ASYNC flag
to switch that new flow.

Flip this flag to enable this mechanism. If it need
to get back to previous sync behavior, we can revert
this change instead of CL[1].

[1]: Ic584203c1221fbae17f5e2d8f09e3992df061646

Bug: 185928120
Test: atest CtsInputMethodTestCases
Change-Id: I2d9600973b34c9d4818e96a8cef771b8a33a07ef