Don't crash TileService app when systemui crashes

Prevent the host app from crashing if systemui crashes while binding to
it. Instead, just return null and let the new systemui process bind
again, as the service will be unbound.

Test: atest FrameworksCoreTests:TileServiceTest
Test: manual, tiles still work
Fixes: 168756844
Change-Id: I70e0e8a223a84874415f7e649106398a5e039097