Fix race condition causing CTS failures: bug 6951063

android.widget.cts.AutoCompleteTextViewTest#testPerformCompletion was
failing intermittently. The cause turned out to be a race between the
request to show the dropdown and the button presses to select the item.
Before the fix, fails on 4/10 runs (on Galaxy Nexus), after the fix
no fails seen.

Change-Id: Idc13ae4cd154320803d19ee2eca192fa4ac6a1dd