Add get/set user selected outgoing phone account APIs.

Promoting getUserSelectedOutgoingPhoneAccount to a public API accessible
to the default dialer app, as well as to system priv apps such as the
settings app.

Promoting setUserSelectedOutgoingPhoneAccount to a system API accessible
to apps such as the settings app.

Add telecom shell commands to support changing the outgoing phone account
in CTS tests.

Bug: 119227574
Test: Manual
Test: Added and ran new CTS test
Change-Id: I204b2f01a1b6746aa35faf8aaacd30ebf53b9a5d