Expand call/connection extras API.

Currently, connection extras are propagated up to Telecom as an
entire bundle.  This is not ideal, as any time a change is made to
the extras, the bundle needs to be fetched, changed, and then re-set on
the connection, where it is parceled to Telecom as a whole.

Using how extras on an Intent as inspiration, this CL adds separate
putExtras, putExtra, and removeExtra methods to allow manipulation of
the extras bundle without operating on it in its entirety.

This Cl also adds support for Calls modifying the extras bundle, with
changes propagated back down to ConnectionServices.

Bug: 27458894
Change-Id: I152340a3bca2dc03f170b06b172a6823410fb961