[WindowDecor] Move WindowDecorTaskResourceLoader calls to main thread

This CL makes the following changes:
- Combine getName and getHeaderIcon since they are always called
  together by their callers
- getNameAndHeaderIcon and getVeilIcon are now run on the main thread
  instead of the background thread. If the resource is not cached,
loadAppResources is run on the background thread. The result is then
returned on the main thread.

This eliminates the following race condition:
1) WindowDecor created, relayout called for the first time and schedules
   the loading of resources (including checking the cache) in the
background thread
2) WindowDecor destroyed while the callback for loading the resources
   has not finished
3) When the callback is finally run, the WindowDecor no longer exists,
   and checkWindowDecorExists throws an IllegalStateException

Test: atest WMShellUnitTests:com.android.wm.shell.windowdecor
Bug: 425315974
Flag: EXEMPT bugfix
Change-Id: Icad26c7641bb4d890318614e77f768d23b9c5d7b