Adds new API to retrieve statsd metadata.

This API can be used by clients to gather stats about statsd, eg.
memory usage, number of metrics/matchers, etc. This data can be used
to debug if devices are not providing expected metrics. The metadata
will be for all configurations, but will not contain the actual
collected metrics since those might have privacy implications.

Test: Tests that statsd and Android still build.
Bug: 69522276
Change-Id: I8e0fedc142f5deed7be6e6309f9444e67d8369ce