Start using ContextImpl, ActivityThread, LoadedApk.

For now, ContextImpl is only used for the "system context". We still use
RavenwoodContext for all other Context's.

Also introduced "RavenwoodAppDriver" which drives app-lyfecycle
related stuff, and moved various existing logic to it.

Bug: 418786021
Flag: TEST_ONLY
Test: $ANDROID_BUILD_TOP/frameworks/base/ravenwood/scripts/run-ravenwood-tests.sh -s
Change-Id: I7b3c0fc2a7fa9b8ed97665f4e74bfe9a1ca110b2