Add IME switch button long click support

This adds support for handling long clicking on the IME switch button.
Initially it will behave exactly like short clicking, i.e. show the IME
switcher menu.

As the IME navigation bar didn't previously support long clicks, this
follows along the logic of the SystemUI navigation bar, with a small
modification: holding down the button for longer than the long click
timeout does not send a cancel event, to maintain the current behaviour
that holding down the IME dismiss button should still hide the IME.

Flag: android.view.inputmethod.ime_switcher_revamp
Test: atest
  NavigationBarTest#testImeSwitcherClick
  NavigationBarTest#testImeSwitcherLongClick
  InputMethodServiceTest#testBackButtonClick
  InputMethodServiceTest#testBackButtonLongClick
  InputMethodServiceTest#testImeSwitchButtonClick
  InputMethodServiceTest#testImeSwitchButtonLongClick
Bug: 311791923
Change-Id: I633da5b03fa50ec1ccf515a4df5538e397d15442