Expose getUserSelectedOutgoingPhoneAccount for priv app or dialer.

Expose the getUserSelectedOutgoingPhoneAccount method for use by
system apps, or the default dialer.  This supports use within the settings
app.

Bug: 119227574
Test: Manual
Test: Added and ran new CTS test
Change-Id: I543b8a59be178a6e3d54a4ed8b100978f475dabf