Removes existing one when apply a default preferred app

Preferred apps are currently parsed from preferred-xml in different
partitions. Always removes existing one when apply default preferred
activity to resolve possible conflicts in different preferred-xml
files.

Bug: 141254650
Test: Push a preferred-xml including a confict to another partition
Change-Id: I5f8bffeaa653ed9b5a5f22c99b8ef6ae0df47493