Handle user context switch in CustomTileMapper

There was a null pointer exception when user context could not be
retrieved. This CL handles that case gracefully.

Bug: 356293227
Flag: com.android.systemui.qs_new_tiles
Test: atest CustomTileMapperTest
Test: atest PlatformScenarioTests
Change-Id: Ieb0905dda4fd0791b4d8663c1f26c1a3ef9524f9
2 files changed