Native puller API: libstatspulled

Create a new library for the native puller callback API. The library has
2 main interfaces: register_stats_pull_atom_callback, which is used to
register a callback, and stats_pull_atom_callback_t, which is the
callback for the puller. It also exposes pulled_stats_event_list, which
is a struct wrapper around a vector of stats_event.

Test: make libstatspull
Bug: 142969725

Change-Id: I14fe4dc523d6f81fdeaa3fe6fad3e95aeecfe92e