Fix binder leakage and activity leakage in MobileNetworkSettings.

There's a binder leakage in MobileDataPreference. Its contentObserver
was only unregistered in onPrepareForRemoval, which is not called when
the activity is destroyed. So adding dispose API to make sure it's
unregistered.

Bug: 80275945
Test: unittest and stress test
Merged-In: Iaa447b46474a2235487aac3c200bba45924e5121
Change-Id: Iaa447b46474a2235487aac3c200bba45924e5121
2 files changed