start/invalidate selection actionMode asynchronously

Text selection is now started/updated using AsyncTasks that first get
a result from the TextClassifier before actually starting/invalidating
the actionMode on the UI thread with the result.

Bug: 34778899
Test: cts-tradefed run cts-dev -m CtsWidgetTestCases -t android.widget.cts.TextViewTest#testSmartSelection
Change-Id: Ie7eda04bb64335744d88b8874363d46af4f94742