Use an Optional wrapper in TileLifecycleManager

This way, we can prevent NPE, as the variable is never null.
Aditionally, make sure that we always perform operations directly on
Optional.

It's hard to test the NPE because of thread issues, but with this fix we
can guarantee:

* No NPE thanks to Optional
* Behavior is maintained thanks to CTS

Test: atest TileLifecycleManagerTest
Test: atest CtsTileServiceTestCases
Test: atest CtsSystemUiHostTestCases
Fixes: 317014639
Flag: NONE
Change-Id: I27a5e5b4ced1fe5da9b0e4b686cb5e58ca820c8c