Use clean calling identity to clear preferred activities

In some cases we need to clear preferred activities for all users after
changing mimeGroups via PackageManager#setMimeGroup. This can result in
clearing preferred activities for non-current user. Which will cause
SecurityException if done without using clean calling identity.

Test: multiple users on device; atest CtsDynamicMimeHostTestCases
Bug: 161239297
Change-Id: I0a84702526ea5ab110aa44056d0407d106d079e3