Load window decor's app name/icon in the bg thread

Adds a WindowDecorTaskResourceLoader class that loads and caches the
app's resources shared across window decoration UI (App Header, menus,
resize veil, etc). The cached resources are coupled to the window
decor's lifecycle and user changes.
The loader util itself is synchronous, so callers are the ones managing
the switching to the bg thread and back.

Bug: 360452034
Flag: EXEMPT refactor
Test: manual - window decor UI loads as intended, perfetto trace shows
binder calls for ActivityInfo and Drawable loading are done in shell.bg

Change-Id: If85ee04d150467471741ac163fd558b0861d555d