Merge restartInput into startInput in internal IPC

As a preparation to start tracking all the event flows that
cause InputMethodManagerService#setImeWindowStatus(), this CL
merges an internal IPC method IInputMethod#restartInput() into
IInputMethod#startInput() in favor of simplicity.

Test: Done some quick manual testing to make sure that IME is
      still working
Bug: 35079353
Change-Id: Ifda6f74ac1b1370d9e9a9fe60354b692121fdcb9