Migrate to using the PhoneSwitcher Handler for SubscriptionChanged

Move off the extra handler onto the main handler for the
OnSubscriptionsChangedListener to avoid using the internal
method and support lazy initialization of the
OnSubscriptionsChangedListener.

Bug: 278814050
Test: atest FrameworksTelephonyTests
Change-Id: I909a1d93dcb69de405f41c5e42a0f78032e08d9d