Better handle MSIM DDS changed event

On DSDS MSIM devices, when Default Data Subscription event
is received, the ImsService may choose to switch which
Slot ID is active.

This change better handles that switch and removes a
crash that was happening due to
ImsServiceProxy#getFeatureStatus getting called
circularly.

Bug: 37361882
Test: Run Telephony unit test
Change-Id: If61332d8c2d4dacb96d27d706abac6afc612749b