Always send WindowToken when starting IME input

In InputMethodManagerService, windowGainedFocus() is a superset of
startInput().  If the window specified to windowGainedFocus() is
already seen in IMMS#mCurFocusedWindow, windowGainedFocus() just works
like startInput().  If the window specified to windowGainedFocus()
does not match to IMMS#mCurFocusedWindow, then it takes care of
everything we need to change the IME target window including tasks
that startInput() does.  The reason why we still have startInput() is
just because my previous CL [1], which unified windowGainedFocus() and
startInput() with keeping the existing behavior, did not intend to
change the existing behavior.

Now that we are going to enforce true traceability between input
session and target application window, IMMS#windowGainedFocus() is
what we want to always rely on.

Bug: 110531072
Test: atest CtsInputMethodTestCases CtsInputMethodServiceHostTestCases
Change-Id: I24feedf9ef5aca90ef947f4f2aa3e78365fd8c8a