Revert "Use mContext directly"

This reverts commit dd3737611c.

Test: None, it's a revert
Bug: 138952436
Change-Id: Ib809ba017e3563c09ea35b1a9056a6a95f4bdfc2

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)
-                     mContext.getSystemService(Context.POWER_SERVICE);
+                     mInjector.getContext().getSystemService(Context.POWER_SERVICE);
             pm.reboot("Checkpoint commit failed");
         }