Create all variants of stubs for statsd

Add separate publicapi, systemapi and module_libs stubs for statsd, and
use a naming scheme consistent with other modules.

Bug: 147768409
Test: m
Test: m framework-media-stubs-{public,system,module_libs_}api
Change-Id: I2b27d27fc7b50a09322a1f5e601def54e6e4ef7a