Floss: Add relevant events for Bluetooth Telephony

Add relevant Bluetooth telephony events and notify CRAS
when these events occur.

This patch enhances the existing on_telephony_use callback to
on_telephony_event, expanding its functionality beyond signaling
uhid open/close events. The new callback can signal more telephony
events (e.g. active_call, answer, reject) to CRAS.

Note: this patch must be merged together with crrev/c/5541553 when
it is cherry-picked back to chromeos as this change affects the
D-Bus API between CRAS and Floss.

Bug: 329015732
Test: Conduct the following manual tests
  pair Bluetooth Headset
  login Ring Central and place an incoming-call
  answer the call on Ring Central
  hangup the call from headset
  check the log with cras_test_client --dump_bt
Test: atest bluetooth_test_gd
Tag: #floss
Flag: EXEMPT, no behavior change on Android; Floss-only changes
Change-Id: I1a3da58fa833fc212d2132b8dc83b4fa4824fe08