Remove @SystemApi from inherited methods of AudioFocusInfo

Since the inherited methods hashCode and equals from the class Object is
already a public API, they don't need to be marked as SystemApi; they
haven't been appear in system-current.txt.

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: I40d60ed524d660f47b5c569980c28c6f0bd83db9