Add memtrackproxy system server service

Serve memtrackproxy service from system server.

This service enforces access control on requests for memory usage stats
to the memtrack HAL based on the call context's PID and UID.

Bug: 177664629
Test: memtrackproxy_test
Change-Id: I10829ffcf4e5b2813d4c622268b0b59da6f171b6
Merged-In: I10829ffcf4e5b2813d4c622268b0b59da6f171b6