Change Resources.getDrawable nullability

Resources.getDrawable() was annotated @Nullable because there was one
very particular path where Bitmap decoding would fail and trigger a null
return.

As part of the switch to ImageDecoder, that path was changed to now throw
an IOException, which will result as a NotFoundException to the caller of
getDrawable.

This CL annotates that path as @NonNull to reduce pain of dealing with
@Nullable method that was very unlikely to be null in practice.

Also fixes many other missing nullability annotations, and relabel
many @Nullable paths that would never return null in practice as
@NonNull.

Bug: 69543526
Test: ResourcesTest

Change-Id: Ib01eca970c5c9969998ce5b265b120aa7048b41a