Moving recent tasks and images to /data/system_ce

Moved user sensitive recents data to the credential encrypted directory
/data/system_ce. Since this directory is not available before the user
is unlocked, and unavailable task ids are needed to be known before
assigning an id to a new task, this change adds a file in /data/system_de
for each user, to store the ids of the tasks present in /data/system_ce.

Bug: b/24569398
Change-Id: I340d1abe23e2594e2cee4bff89a697bd8dc61fc7