Refer to STAY_ON_WHILE_PLUGGED_IN via the Global namespace

It moved from System to Global, so writes are not automatically redirected
to the new namespace (else apps would start crashing).

Bug 7126575

Change-Id: Ief31fcb5a6107a098da04d30d146e16921dee776

Author: Christopher Tate

services/java/com/android/server/DevicePolicyManagerService.java

@@ -1628,8 +1628,8 @@ void updateMaximumTimeToLockLocked() {
             } else {
                 // Make sure KEEP_SCREEN_ON is disabled, since that
                 // would allow bypassing of the maximum time to lock.
-                Settings.System.putInt(mContext.getContentResolver(),
-                        Settings.System.STAY_ON_WHILE_PLUGGED_IN, 0);
+                Settings.Global.putInt(mContext.getContentResolver(),
+                        Settings.Global.STAY_ON_WHILE_PLUGGED_IN, 0);
             }
 
             mLastMaximumTimeToLock = timeMs;