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.

Flag: android.view.inputmethod.ime_switcher_revamp
Test: atest
  TaskbarNavButtonControllerTest#testPressImeSwitcher
  TaskbarNavButtonControllerTest#testLongPressImeSwitcher
Bug: 311791923
Change-Id: Ibd0d6bce2bab11511bc082f46731ec86038c8cf8