Migrate AutoCompleteTextView to use OnBackInvokedDispatcher.

With ag/17952046 we now actively intercept KEYCODE_BACK before they are
sent to IME when new back flow is used. This requires us to migrate
AutoCompleteTextView#onKeyPreIme, and is also what causes the test
failure in b/231960630.

Test: atest AutoCompleteTextView test.
Test: Test back behavior of AutoCompleteTextView in BackTestApp.
Bug: 231960630
Change-Id: I1a5e01c9a4f8f0e762b493284650854a230aecd3