Add anon RSS + swap metric

We are planning to use this metric to detect leaks.

This CL also decouples the actual memory sampling from AM. This means:
- Less time locking the pid list (we used to lock and then read proc)
- Less serialization / deserialization for the parcel
- Simpler to evolve (e.g. removed the HWM-specific method in AM)

Change-Id: I87a7243156dd8c88cfa85038e7e6cf4963e271e1
Test: manual, MemoryStatUtilTest, UidAtomTests
Bug: b/135418017