Don't return null in the default ActivityManagerService.Injector.

Usually with injectors like these, we are expected to use methods
provided by these instead of directly accessing the fields, so just
returning "null" in Injector.getContext() is counterintuitive and
ideally other callsites which are directly accessing mContext should
be updated to use injector provided one instead.

Bug: 138952436
Test: atest services/tests/servicestests/src/com/android/server/am/ActivityManagerServiceTest.java
Test: atest services/tests/mockingservicestests/src/com/android/server/am/AppCompactorTest.java
Test: atest services/tests/servicestests/src/com/android/server/am/AppErrorDialogTest.java
Change-Id: Idb7cb9a5b7c4c0d237ece9716932a260956a0109