Support atom-level annotations within AStatsEvent

Refactors implementation details to allow for atom-level annotations.
This CL does not change the API surface. addBoolAnnotation and
addInt32Annotation work for both atom-level and proto field annotations.

Test: bit libstatssocket_test:*
Bug: 151158794
Change-Id: I2c340b201aeb3bcddd5cfde171b7c4df1d8d69a4