Offer Mainline stubs under Ravenwood.

Mainline hidden APIs can change at any time, so we don't want to
bring them across as-is, so `all-updatable-modules-system-stubs`
is used instead to ensure we only bring across published stub APIs
that module authors intend to be stable.

Bug: 319647875
Test: atest SystemUiRavenTests
Change-Id: Ib0837ee65ce125f27e0e7f3da6effe814065c35b

Ravenwood.bp

@@ -81,6 +81,7 @@ android_ravenwood_libgroup {
         "framework-minus-apex.ravenwood",
         "hoststubgen-helper-runtime.ravenwood",
         "hoststubgen-helper-framework-runtime.ravenwood",
+        "all-updatable-modules-system-stubs",
         "junit",
         "truth",
         "ravenwood-junit-impl",