Expose phone account register/unregister intents.

These were previously @SystemApi.  Retaining the existing SystemAPI
behavior which sends the intents to those with a private permission.
Extending to ALSO send these intents to the default dialer app as well
using an explicit intent.
Cherry-pick from aosp-master to resolve merge conflicts.

Test: Manual
Bug: 37106957
Change-Id: Ifb72870105be5ba024af196a8c3165a9afb397ab