Removing hidden api usage from AutoCompleteTextView

ensureImeVisible() was added to show the keyboard in SearchDialog go/ensureImeVisible-added. A better fix would be to expose a function to setInputMethodMode(), as showDropDown() is public and can be called externally. Adding this new API in this CL.

Bug: 123768913

Test: Added tests to android.widget.AutoCompleteTextViewTest.

Change-Id: I07e53c6d5e93d1e33381f8f87f75a7dee2f2f5a5