Lazily init Handler in SubscriptionManager

Convert to lazy initialization of the Handler inside the
OnSubscriptionsChangedListener to avoid the undocumented
requirement that it always be created inside of a thread that
has a prepared Looper.

Bug: 278814050
Test: atest FrameworksTelephonyTests
Test: atest SubscriptionManagerTest#testCreateSubscriptionChangedListenerWithoutLooper
Change-Id: I22fec4924ed7299f415e0cbf2c208e65a4f2e3d1