In onDragEvent(), don't assume text is Spannable

Previously, onDragEvent() tried to set the anchor even if
the text was not Spannable. Now we check to make sure it is
Spannable before trying to set the selection.

Test: cts-tradefed run cts-dev --module CtsTextTestCases
Change-Id: I835bf3d6024bf3c85e1d248458829eef496ad93d
Fixes: 37261326
1 file changed