Provide a system_server internal API to get isolated PIDs of an UID

Internal services within system_server can query the PID list of
the isolated processes with packages matching the given UID.

Also added a shell command to help the testing.

Bug: 191703385
Test: atest FrameworksServicesTests:ActivityManagerTest
Change-Id: I9749bc9c70902221f41945c7e785d13631acbae2