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
Merge "Rename domain "system" to "system_server"."
Merge https://android.googlesource.com/device/lge/mako