Pinning components in ChooserActivity

Move ChooserActivity to SystemUI. This is a safer place for it to live
and still be able to persist data to storage.

Add a context menu to long press for chooser targets allowing users to
'pin' a target component from an app. This causes it to sort to the
front of the list so that a user's favorite apps are always available
from share UIs, etc. Similarly, all ChooserTargets from a pinned
component receive an impossibly large boost for sorting so that they
will always appear first.

Bug 26791843

Change-Id: Ib4e603d9d4263403e98ce619287452ddab593044