libsnapshot: Add more feature flags to SnapshotMergeReport.

This patch also begins reducing the complexity of SnapshotMergeStats by
eliminating the indirection layer between the protobuf and
SnapshotManager.

Bug: 222117189
Test: statsd_testdrive
Change-Id: I15d740121c381da7d8311f0cbbd0da82db877555