Move copy, cut and paste shortcut handler to onKeyDown().

The shortcut handler for Shift-Delete was added in onKeyShortcut(), but
it's called after onKeyDown() and the event are consumed by
BaseKeyListener before reaching to onKeyShortcut().

Fix: 122079351
Test: atest CtsWidgetTestCases:android.widget.cts.TextViewTest
Change-Id: Ifee0144bdd3c4f0552bc4c4f3e5b844502bc937e