Introduce IME switcher revamp API flag

Following up from [1], this introduces a new IME_SWITCHER_REVAMP_API
flag, to move the API changes of the feature under a separate flag.

  [1]: I18e7c5dc5271bee44e4540b3070b989df4e27c4c

Test: atest
  InputMethodInfoTest#testEquals
  InputMethodInfoTest#testInputMethodInfoProperties
  InputMethodInfoTest#testInputMethodInfoWriteToParcel
Bug: 311791923
Flag: android.view.inputmethod.ime_switcher_revamp_api
Change-Id: Ibb4158fffc00666e295b93adc2542024416f4f2c