Add content description to tile

The tile in TileRequestDialog didn't have a content description. This
made it so that some screen readers could pick that there was text and
read it, but others (like select to speak) would not.

Fixes: 236174008
Test: manual, using Select to Speak
Test: atest TileRequestDialogTest
Change-Id: I89e3819413a9d8dcf0345ce43fb5594b6a8ffb0e