Hide cross-user getInstance() and promote non-cross-user getInstance()

getInstance(Context, String, UserHandle) is only useful for privileged
components with INTERACT_ACROSS_USERS_FULL like SysUI and Settings,
which can access hidden APIs. Therefore, there is no advantage in
exposing the method as a public API and we can keep it hidden.

As an alternative, this change promotes getInstance(Context, String) from
SystemAPI to public API, which supports all the use cases required by
public API users.

Bug: 288580225
Test: Presubmit.
Change-Id: I08372cd87b46f54303cd32055c96d20953b9fc6d