Fix IME be dismissed abruptly

CL[1] used to fix some IME animation issue.
And simplified the check focused view logic.

But when window focus changed, the focused view
may still be null. So ImeFocusdController use the
DecorView to start an new Input and hide keyboard.

Use the focused view after focus change callbacks
when we call onPostWindowFocus as before.

[1]: Ib140801f1ce03b5566e756914f96dacba3ad8892

Bug: 186331446
Test: Manual test with the bug steps
Test: atest FocusHandlingTest#testRequestFocusOnWindowFocusChanged
Change-Id: I2f8fde0b6575db17955ff8b8804b61378f9d6dad