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
(cherry picked from commit dd3737611c640a764bcc55d8bcea1d13c7ea7989)
diff --git a/services/core/java/com/android/server/am/ActivityManagerService.java b/services/core/java/com/android/server/am/ActivityManagerService.java
index 91ae156..88c2998 100644
--- a/services/core/java/com/android/server/am/ActivityManagerService.java
+++ b/services/core/java/com/android/server/am/ActivityManagerService.java
@@ -5259,7 +5259,7 @@
             storageManager.commitChanges();
         } catch (Exception e) {
             PowerManager pm = (PowerManager)
-                     mInjector.getContext().getSystemService(Context.POWER_SERVICE);
+                     mContext.getSystemService(Context.POWER_SERVICE);
             pm.reboot("Checkpoint commit failed");
         }