5/ Update splitscreen to use shell main thread

- Remove some usages of handler
- Remove separate executor for transaction updates (until it's clear
  we need to offload them)
- Only animations running on the main shell thread are using the animator
  handlers, so repurpose that to be for the main thread until we need it
  for things that actually run on the animation thread (for transitions)

Bug: 161979899
Test: atest WMShellUnitTests
Test: atest SplitScreenTests
Change-Id: I0ea9aae2deddafb620f1f41658e8366443bf9aaa