Prevent NPE in CustomTile

The icon is passed by the TileService, so it could null when retrieving
the drawable. Check for null before doing operations on it.

Test: atest CustomTileTest
Fixes: 140479361
Change-Id: I0ee11806c0047e0e524f5201d369bee51beba451
Merged-In: I0ee11806c0047e0e524f5201d369bee51beba451
(cherry picked from commit cfdd8d40)