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
diff --git a/BoardConfig.mk b/BoardConfig.mk
index e173fba..054360f 100644
--- a/BoardConfig.mk
+++ b/BoardConfig.mk
@@ -112,7 +112,7 @@
        nfc.te \
        rild.te \
        surfaceflinger.te \
-       system.te \
+       system_server.te \
        ueventd.te \
        wpa.te