Move resources loading from Compose to UiBackground thread.

Compose has some known issues when loading resources (specially icons).
This change moves some of that stuff to UiBackground thread instead of
dealing with it in Compose.

Flag: com.android.systemui.volume_redesign
Fixes: 399780453
Test: atest VolumePanelScreenshotTest
Test: manual on the foldable. Observe that the sliders change and
voiceover anouncement is the same as before.

Change-Id: I429a8254db96c83325fd42d68b93e2d385073391