Wait for the disconnection of ePDN to exit ECBM

When exiting emergency callback mode, ePDN Disconnect procedure can
be aborted since quick rollback is ongoing after re-enabling NR by
exiting ECBM. This causes failures in certficate test of some carriers.

Bug: 299866883
Test: atest EmergencyStateTrackerTest
Change-Id: I7432a666f74008cb4b054f05a9f10f6a5ca289e3