Add connection events for indicated when a connection is remotely held.

Note: This is different than the existing EVENT_ON_HOLD_TONE_START
and EVENT_ON_HOLD_TONE_END events, which trigger playing a hold tone for
remotely held calls.  Those events also take into account whether the call
is foreground/background; it is left as a future exercise to clean that
up.

Bug: 30349182
Change-Id: I919365d243ae3b9fe56454c7a2b9d5805b2c03a2