Add accessibility labels for IME switcher button

This updates the content description of the IME switcher button, and
adds an accessibility label for the long click action.

Additionally marks views that are not important for accessibility.

Test: enabled TalkBack and observe
Bug: 311791923
Bug: 355594403
Flag: android.view.inputmethod.ime_switcher_revamp
Change-Id: Icd59d50887bb5812f493a4e5070106fb92ad4d3e