Keep a strong reference to the InlineContentCallbackImpl

1. Keep a strong reference to the contentCallback.
2. Make sure inflated() only allowed to be called once.

Bug: 146524826
Test: atest CtsInputMethodTestCases:android.view.\
inputmethod.cts.InlineSuggestionTest
Test: manual. Call inflate() twice and make sure the
behavior is expected.

Change-Id: Id9bd386269111d11a2b2c27e33832cac90061b61