Rename ANDROID_RUNTIME_ROOT to ANDROID_ART_ROOT following APEX module name change.

Test: Boot
Bug: 135753770
Change-Id: Ie9a8043ee350164c01a66f1a7ab1b3205076a5c4
diff --git a/rootdir/init.environ.rc.in b/rootdir/init.environ.rc.in
index 17f6596..50005d9 100644
--- a/rootdir/init.environ.rc.in
+++ b/rootdir/init.environ.rc.in
@@ -5,7 +5,7 @@
     export ANDROID_ASSETS /system/app
     export ANDROID_DATA /data
     export ANDROID_STORAGE /storage
-    export ANDROID_RUNTIME_ROOT /apex/com.android.art
+    export ANDROID_ART_ROOT /apex/com.android.art
     export ANDROID_I18N_ROOT /apex/com.android.i18n
     export ANDROID_TZDATA_ROOT /apex/com.android.tzdata
     export EXTERNAL_STORAGE /sdcard