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