Fix re-entrant issue in Keyguard / Mixed handler

We need to:

- Increment mInFlightSubAnimations optimistically before startAnimation
  because it might immediately call back before even returning true,
  which is allowed. If it doesn't do this and returns false instead, no
  problem; we'll just decrement it again.

- Post finish callback from Keyguard's "Remote" Transitions (which are
  just remote for API compatibility and execute synchronously in the
  same process) so that housekeeping from startAnimation can be
  guaranteed to finish before the onFinish callback comes back around

Bug: 286242775
Bug: 286507622 # to be confirmed
Test: atest WMShellUnitTests:ShellTransitionTests
Test: atest KeyguardTests
Change-Id: Ida3035f1a99992a1309033ffdd7498298e44e81f