Make APIs to get system phone accounts public

This is used by Wi-Fi calling to handoff and incoming
call to a cellular call.

For example, when doing an handoff we can look through
the outgoing phone accounts and find the one that the user
has verified with their google account. We can then
handoff to that phone account.

Bug: 17686424
Change-Id: I06d42aa8f4a36d11e5293ed6f7c62b796226f5c1