DO NOT MERGE Inform registrants of which connection started or stopped the hold tone.

When receiving a hold tone signal and informing registrants via
startOnHoldTone/stopOnHoldTone, also include the connection which initiated
the start/stop. This is necessray to ensure only the TelephonyConnection
which initiated a hold tone issues a Connection Event.

Bug: 25357778
Change-Id: I1590b61affb28c40173f08a8f6233d8efe6a5e6e