BinaryTransparencyService: Add APEX update observer

Previously, an observer for APK updates was added/registered.

In this change, we add an observer for staged APEXs so that
our coverage is more complete. This should help the case where
rebootless APEX updates happen.

Other minor adjustments:
- refactor all package update hooks into helper methods and change
  its call from onStart to onBootPhase.
- refactored logic in onBootPhase such that execution returns if
  it is not in PHASE_BOOT_COMPLETED to eliminate large if clause
  for better readability.

Bug: 264428429
Test: Manual.
Change-Id: Ib54daafad47ea6f2eca8fe2470d8edda007214d3