Cache default home

Bug: 162102512

Cache the default home app (package) in the slice manager.  The cache is
invalidated whenever the HOME role is changed.

Test: Tested with a debug build that computed the cached value and then
compared it to the original computation.  Verify that no discrepancies
are detected.
 * Manual test -
   1. Install the 3P Nova Launcher app.
   2. Choose Nova Launcher as the default home app
   3. Choose Pixel Launcher as the default home app
   4. Choose NOva Launcher as the default and then uninstall the app
 * FrameworksServicesTests:UserSystemPackageInstallerTest
 * UserLifecycleTests

Change-Id: I8e31acced3b41262641c522842e5f553c63af102