Test untested ResourcesLoader APIs

loadFromApk and clearProviders were not tested within the resources
loader tests. This change adds tests for both of those APIs, adds
tests to ensure loaders work correctly when some loaders have values
defined for resources/assets and other loaders leave values undefined,
and adds tests for cookie-based assets APIs.

Bug: 152617337
Bug: 152617891
Bug: 151949807
Test: atest CtsResourcesLoaderTests
Change-Id: Ib4c1ac50749720233917e721d26b815d0ceb50af
7 files changed