Further refactoring of SafetyCenterResourcesContext.

Stop making it implement ContextWrapper, that's error-prone
as it implies it has the Context from the resources APK as a base
when in fact the base is the context passed in; and methods are
overridden only partially.

Also increase test coverage.

Bug: 283100177
Test: atest CtsSafetyCenterTestCases
Relnote: N/A
Change-Id: I2925921de3de01633beacbbe713d46e9522f003b