Add type to filter option to list view

Make ListAdapter implement Filterable. Add new implementation for Filter
to search based on prefixes.
When creating the list include the filter option to create actual list entries.
Since Filter.performFiltering is done on a work thread, introduce a new lock
and a HashMap of packages matching a prefix which is then used to create a filtered list of applications.
Also separte out modifying mAppLocalList which is the basis for list and
the dependent mFilterMap used by Filter.performFiltering method to Locked
methods for clarity. Note that mAppLocalList is manipulated on main UI thread everywhere but mFilterMap is not.