Created new @TestApi (and Shell command) to get policy-exempt apps.

Test: adb shell cmd device_policy list-policy-exempt-apps
Test: atest FrameworksServicesTests:com.android.server.devicepolicy.DevicePolicyManagerTest#testGetPolicyExemptApps_noPermission,testGetPolicyExemptApps_empty,testGetPolicyExemptApps_baseOnly,testGetPolicyExemptApps_vendorOnly,testGetPolicyExemptApps_baseAndVendor

Bug: 181238156
Fixes: 182373142

Change-Id: I990c53d1a2b9a6e15ad9fcae310badb7aae834d1