Add support for busy number simulation in Telecom test app.

Its difficult to get a number to be busy nowadays as everything always
rings through to voicemail.
This change adds support for dialing a number ending in BUSY through the
Telecom test app to have the outgoing call end with a BUSY disconnect
cause.

Test: Manually dialed 555-BUSY in test app to verify functionality.
Bug: 78340691
Change-Id: I9f6730b9baf2f6a54c795105ab5589134f74efbb