Statsd and frameworks reference proto enums

For frameworks constants that don't have intrinsic meaning (i.e. their actual
value and order don't matter), so that it is unlikely that their values
will be changed:
This cl introduces proto enums representing some constants found in
the Android codebase, and connects the two.

By using the Proto enum as the source-of-truth, it means that Java and
proto can be kept in sync. Otherwise, when the Java frameworks code
changes, it silently breaks the protos from working properly, since the enums
are wrong. By having the Java code reference the proto enums, it ensures
that everything is in sync. The values of the constants are unchanged.
But future changes to these constants will need to be done in the proto
file, which the Java file merely references.

The protos are necessary for incidentd and statsd and, in the future,
possibly dumpsys. In this way, the logging mechanism is much less likely
to get broken when new constants are added, and we can be ensured that
the logging accurately reflects the underlying codebase.

Bug: 69478930
Test: cts-tradefed run cts-dev -m CtsStatsdHostTestCases
Test: cts-tradefed run cts-dev -m CtsIncidentHostTestCases
Change-Id: If79032c34b2799db1e3e70cb47b1312fd72092b9