[res] Don't create extra asset provider when not needed

MultiAssetsProvider is useful to combine a ResourcesLoader with
a file-based provider, but most of the time it's used without
any loaders, and ends up with an EmptyAssetsProvider object that
just wastes cycles and RAM.

This CL optimizes that out, only creating the extra layer when
we have a real loader, and falling back to the only remaining
provider otherwise

Bug: 319137634
Test: build + boot + manual + atest libandroidfw_tests
Flag: EXEMPT optimization
Change-Id: Ibc5ac60adc5e008b70a62481a747758c89083ff9