Use mContext directly

We already have the context saved, so we can just use that.

Test: Edited code to force expection path to be called, confirmed
      reboot happens
Bug: 138952436
Bug: 140111727

Change-Id: I40e95d72c63f864131f0028c47f121463d0f6dff

services/core/java/com/android/server/am/ActivityManagerService.java

@@ -5259,7 +5259,7 @@ public class ActivityManagerService extends IActivityManager.Stub
             storageManager.commitChanges();
         } catch (Exception e) {
             PowerManager pm = (PowerManager)
-                     mInjector.getContext().getSystemService(Context.POWER_SERVICE);
+                     mContext.getSystemService(Context.POWER_SERVICE);
             pm.reboot("Checkpoint commit failed");
         }