Added a filegroup for statsd config proto definition.

This is used in other parts of the framework for serializing some
statsd text proto config at build time. The filegroup allows aprotoc
to access the definitions elsewhere in the source tree.

Bug: 120569785
Test: Build is not affected.
Change-Id: I82a5f8869a2d05ea27d205f471b9c5b5b875fefc
Merged-In: I0e37a9b9a1f22a5dcebbf07ccc5cad98cf76024c