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 tests
Merged-In: I55cac364221c728ad1c77be783c358e8d98ec2b4
Merged-In: I68cd04f288f2d7006a2c55cba3584fd74117a1b8
Change-Id: Iaa29bba25f82e699c96b951b3b110568163e12d8