Switch OnSubscriptionsChangedListener to use existing Handler

Use the main Handler for DevicePolicyManagerService to post
the callback for OnSubscriptionsChanged to move away from
using the hidden (internal) handler.

Bug: 278814050
Test: atest FrameworksTelephonyTests
Test: atest SubscriptionManagerTest#testCreateSubscriptionChangedListenerWithoutLooper
Change-Id: I347408af8cd6cbcd3a25facc056c7c2541d5b428
1 file changed