The rationale for this change:
1. When defining enum values for platform, we want to use the current
integers, in some cases zero is not defined, but proto3 enforces a zero
default value.
2. Android Metrics Team uses proto2 on server-side
3. When copying .proto to server-side, the known issue of dropping
unknown fields might affect if using proto3
4. Not much benefits from using proto3
Bug: 67110257
Test: manully generate incident report and it looks normal
Change-Id: Ia63e39de549a46683e9f80fcb74f1d771782b7f4
c7f93070b6