Failed to show hold call when user click swap and merge simultaneously

Merge request is failed with exception from framework itself
but EVENT_CALL_MERGE_FAILED is not propagated to dialer,
resulting in clearing the hold banner.

Bug  : 418942310
Test : Manully and UT verified
Flag : com.android.internal.telephony.flags.send_merge_failure_on_handle_conference_failed
Change-Id: I55e24b3cfedc1c6c0d1f45c8be64f7864cbd9dd9