Add uid field annotation in atoms.proto and statd memory usage optimization.

[memory]
  statsd binary size from 664k -> 600k
  memory usage 1978k -> 1813k (with no configs)
  + Avoid initialize any static map in statslog.h to avoid many copies of the map in each include.
    - Do it in cpp so that it is initialized only in places that use them

[Uid annotation]
+ Uid annotation is needed for extracting uid from dimension for UidCpuPuller.
+ After the change, stand-alone uids don't need to be in field 1 anymore.
+ Also added exclusive bit annotation in AppDied
+ Currently only allow one uid field in an Atom. This is to keep things simple until
  we find an exception.

Test: statsd_test
Bug: 73958484
Bug: 72129300

Change-Id: I8a916d5c00d5930e24ae7e0825a57dea19c0e744