Slight refactor of SafetyCenterResourcesContext.

Only cache the resources context, not every field.

Ideally it should implement ContextWrapper for the resources APK
context directly, I may follow up with another CL to do that (it's
a bit more risky and involved).

Also guard it by the API lock to make sure the resources context
is loaded at most once.

Bug: 283100177
Test: atest CtsSafetyCenterTestCases
Relnote: N/A
Change-Id: Ia54faa909105566fd5382c68b3e9046e1628267b