Integrated policy-exempt-apps into setLockTaskPackages().

Test: atest FrameworksServicesTests:DevicePolicyManagerTest
Test: manual verification using TestDPC (cannot test using CTS
      because it require launching an activity provided by the OEM,
      which the test has no control over).

Bug: 181238156

Change-Id: I5c2bfed3b1dcd66cb94461cb5695868b6090219a