[Ravenwood] Support context.getResources() without res APK

When a test uses framework resources without including resources of its
own, the existing implementation will crash the test. Make sure
resources are always available regardless whether the test includes its
own resources or not.

Bug: 339614874
Flag: EXEMPT host side change only
Test: $ANDROID_BUILD_TOP/frameworks/base/ravenwood/scripts/run-ravenwood-tests.sh
Test: atest RavenwoodServicesTest
Change-Id: Ic9000d892d6b899ffbbbecb7d713fee19a601b07