Group annotations for the same atom id

Group annotations for the same atom id inside one if-block in generated
code.

Use shared_ptr to store AtomDecls in multiple data structures.

Store a mapping of field numbers to atoms that have annotations at
corresponding field numbers in Collation.h

Bug: 151744250
Test: stats-log-api-gen-test
Test: m stats-log-api-gen
Test: m libstatsmetadata
Test: m statslog-framework-java-gen
Test: m libstatslog

Change-Id: I874696cfb5c27141017b4293bec809ab510ceb98