Add an API to read experiment IDs.

Change the file format to store the
experiment IDs direclty so we don't
need to parse the proto when reading
the values out.

Bug: 129099771
Test: bit statsd_test:* && adb shell cmd stats pull-source 10051
Change-Id: I0dc1fd118f4d9ba597c2f0959648136bbafb5aab
9 files changed