Add "call" method on ContentProvider.

This permits implementing interfaces which are faster than using
remote Cursors.  It then uses it for Settings & SettingProvider, which
together account for ~50% of total ContentProvider event loop stalls
across Froyo dogfooders.

For fetching Settings this looks like it should reduce average
Settings lookup from 10 ms to 0.4 ms on Sholes, once the
SettingsProvider serves most gets from in-memory cache.  Currently it
brings the Sholes average down from 10ms to 2.5 ms while still using
SQLite queries on each get.