Crash shell if callers use the wrong thread

This prevents data races from custom transition handlers by making the
race very noticeable before anyone gets a chance to ship it.

Test: atest ShellTransitionTests
Flag: com.android.window.flags.enforce_shell_thread_model
Bug: 351189446
Change-Id: I3e9381e7a65a293a35b3e17ea37335f2e52d5e70