Add asynchronous geocoding API

Geocoding is unsupported, but the old APIs encourage bad behavior and
force clients to waste a thread blocking for results. This often causes
ANRs in a variety of applications. Provider an asynchronous API so
clients can avoid ANRs.

Bug: 197585308
Test: presubmits
Change-Id: I6fdd90d467f01892ce34371c2161721bd5976ca9