Merge restartInput into startInput in internal IPC

This is the 2nd attempt to merge restartInput into startInput in
internal IPC after fixing the mistake in new parameter order in
the previous CL [1].

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.

This is a refactoring CL that should have no behavior change.

 [1]: Ifda6f74ac1b1370d9e9a9fe60354b692121fdcb9
      1a5838e9

Test: Set true to InputMethodService#DEBUG and make sure startInput()
      and restartInput() are called in the following scenario.
      1. Complete the setup wizard.
      2. adb shell am start -a android.app.action.SET_NEW_PASSWORD
      3. Proceed to "Choose your password" page
      4. Make sure startInput() gets called.
      5. Type "aaaa" then hit "CONTINUE" button.
      6. Make sure restartInput() gets called.
Bug: 35079353
Change-Id: I476d0cf8cbb0a0134941854f9337d9ad15e66a71