Formalize DisconnectCause API

As part of telecom mainline, formalize the hidden DisconnectCause
constructor (used by Telephony) into a system API. Additionally, the
telephony related debug info getters also need to be formalized:
getTelephonyDisconnectCause, getTelephonyPreciseDisconnectCause,
getImsReasonInfo.

Bug: 322518612
Bug: 311773409
Test: atest DataObjectUnitTests
Change-Id: Idb0c502bde5391012349a58358515c59fc3cb893