SurfaceFlinger: Avoid race in flag manager

Initializing the flag manager from one binder
thread while using it from another doesn't seem
very safe. This is in fact what could happen if
onBootFinished overlaps with dump, the objects constructor
doesn't do anything in particular so we can just default
construct it as a value member instead.

Bug: 219059993
Test: Existing tests pass
Change-Id: I7162812b000d67adb9ed745d31d8bac8b2e8f73e