Migrate OnSubscriptionsChangedListener to different Handler

Remove calls to the internal method of OnSubscriptionsChangedListener.
This causes a minor change to the threading since posts now go through
the same handler twice, but it should be benign and thread safer.

Bug: 278814050
Test: atest FrameworksTelephonyTests
Change-Id: I3081003ec6092334df9dca7833ed53b96d313b98
3 files changed