DSDA: Deflect MMI codes

Reject MMI codes when there's an ongoing call on a different phone
account. Instead, display an error message to the user explaining that
the MMI code is not available when there's already another call in place on another phone account.

Bug: 389787292
Test: atest CallSequencingTests
Test: atest atest CtsTelecomCujTestCases:CallSequencingMmiTest
Test: Manual verification in Dialer UI that error message is shown when dialing incall MMI across phone accounts.
Flag: com.android.server.telecom.flags.enable_call_sequencing
Change-Id: I93b805882da9afabb8afb7657912e703f991c795