Split system_app from system.

system_app is for apps that run in the system UID, e.g. Settings.
system is for the system_server.
Split them into separate files and note their purpose in the comment
header of each file.

Change-Id: I19369abc728ba2159fd50ae6b230828857e19f10
Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>