Remove @SystemApi annotation from private or package-private members

Some private or package-private members have been marked as @SystemApi,
which do not make sense as they aren't accessible to apps due to the
access modifier. Having annotation on such members hasn't been harmful,
but will soon trigger CTS failures when we do the exact match for
SystemAPIs; a runtime entity annotated as @SystemApi have to exist in
the documented API (e.g. system-current.txt). So, removing the bogus
annotations.

Bug: 67891551
Test: m -j does not require m update-system-api
Change-Id: I0083475ef5435cf9a582c2ce1be469c92328fe69