Adds API for apps to push events to statsd.

This API allows app to construct custom metrics based on labels
chosen by the app developers. Also added some buttons to manually
test this functionality in the dogfood app.

Test: Verified that Android can be built and tested with custom app.
Bug: 69522276
Change-Id: Ifb7abea4c1d62fb435a9cb6f32df12bc2234d82f