Fix NPE of ActivityManagerInternalTest

This is a follow up of I634f247e42a7e8e4535eedd76cf84089abdf1129

Bug: 136036078
Bug: 171450666
Test: atest FrameworksServicesTests:ActivityManagerInternalTest
Change-Id: I9d10a644392d8ae4cf2ede7e18f2b77d09f9b00d

services/tests/servicestests/src/com/android/server/am/ActivityManagerInternalTest.java

@@ -62,6 +62,8 @@ public class ActivityManagerInternalTest {
                 .getContext();
         doReturn(mServiceThreadRule.getThread().getThreadHandler()).when(mMockInjector)
                 .getUiHandler(any());
+        final ProcessList dummyList = new ProcessList();
+        doReturn(dummyList).when(mMockInjector).getProcessList(any());
         mAms = new ActivityManagerService(mMockInjector, mServiceThreadRule.getThread());
         mAmi = mAms.new LocalService();
     }