Additional CTS tests for TextView.onDragEvent

These tests confirm that we don't break or end up in an invalid
state if a TextView starts as non-editable and then is updated
to be editable during a drag action.

Bug: 182617122
Test: atest CtsWidgetTestCases:TextViewOnReceiveContentTest
Change-Id: I28513fbc70425befd703aa9651775ab6780f9f1b