Change policy vars to concatenation.

Using assignment versus concatenation doesn't
allow for easy inheritance models.

Change-Id: I53ab882e79966b0491636685c341ae105c472ffd
Rename domain "system" to "system_server".

Now that the "system" domain is just about the system_server, it's
clearer if the domain is called "system_server".

This CL depends on the renaming in external/sepolicy performed in
https://android-review.googlesource.com/#/c/65502/.

Change-Id: I90a6850603dcf60049963462c5572d36de62bc00

Conflicts:
	sepolicy/compatibility.te