DataCallSessionStats captures Cross-SIM calling

- This revert^2s commit 6c57c33c,
- patches the fix on top of this commit, and
- gates the new functionality behind a feature flag.

FIX DESCRIPTION:
- The DefaultNetworkMonitor singleton prevents the memory leak bug with
 multiple default network callback registrations, one per DataNetwork.
- DataCallSessionStats gets a reference to DefaultNetworkMonitor
 on creation, to determine when a PDN is established on IWLAN, whether
 it is WiFi Calling or Backup Calling.

Test: Updated DataCallSessionStatsTest, Live test with flag enabled to
 verify data_call_session proto sets 'iwlan_cross_sim' during Backup
 Calling. Verified no functional regression when flag is both enabled
 and disabled.

Bug: 313956117
Change-Id: I869165a1fecf18a2eeaab18bd3a0b639d1fc8e1b