Add ActivityThread.currentSystemContext().

We want to expose getSystemContext(), but unfortunately it returns
ContextImpl not Context, and we can't change the result type
because of the @UnsupportedAppUsage.

This means we'd need to create another getter with a return type of
Context, which means we'd have to update the callsites. So let's
just add a static currentSystemContext() just like the other current*()
methods, because that's what almost all clients would want to use anyway.

Bug: 402153527
Test: $ANDROID_BUILD_TOP/frameworks/base/ravenwood/scripts/run-ravenwood-tests.sh -s
Flag: EXEMPT refactoring only
Change-Id: I06aaa85bb50a7ebd69910272193ac54263cfcc00