Don't use ApkAssets::GetPath for equality checks

With the introduction of ResourcesProviders, not all ApkAssets have
paths on disk. Theme::SetTo and various AssetManager methods use the
path to perform equality checking on ApkAssets. This equality check
will be performed on the debug string of an ApkAssets if it does not
have a path on disk. This causes ApkAssets with the same debug name
(like "<empty>") to be seen as the same ApkAssets. Rather than using
path, the pointer to the ApkAssets should be used for equality checking
since ResourcesManager caches and reuses ApkAssets when multiple
AssetManagers request the same assets.

Bug: 177101983
Test: atest CtsResourcesLoaderTests
Change-Id: I11f6a2a3a7cc8febe3f976236792f78e41cf07e6