[PackageManager] Add @NonNull on ResolveInfo#loadLabel

Add @NonNull for the method and also add @NonNull on the
parameter (pm). If pm is null, it throws NPE in the original
behavior. We add the new null check will not change the original
behavior. It should be safe and backward-compatible.

Test: atest ResolveInfoTest
Bug: 271814726
Change-Id: Ic4ed1873488facaed29e62a50274b0a95a8baec1