Layer: Use raw pointers for Current/Drawing parent

We should only be reading/writing this from the main thread
and likewise we only delete layers on the main thread and
so using raw pointers and managing the lifetime from the
Layer destructor will be safe. This significantly decreases
overhead in various code that traverses via parent (getAlpha,
isVisible, etc...).

Test: Existing tests pass. simpleperf
Bug: 186200583
Change-Id: I45745f7c865177ddfe9105d1440a9fa8f3470823