Remove StartInputFlags.FIRST_WINDOW_FOCUS_GAIN

Since this flag does not do any check during start input,
Remove this flag and the related parameter for IMM#onPostWindowFocus
to keep the logic simpler (included removing
ViewRootImpl#mHasHadWindowFocus).

This is refector CL and does not impact any behavior change.

Bug: 141738570
Test: Refector CL, make sure all existing test passed.
Change-Id: I9119f4846cbbd2b15246dea9a3b1fc5845dce951