DevicePolicyManager unit test improvements:

- Use JUnit4 and Truth.
- Updated javadoc instructions to use atest.
- Add missing license header.
- Re-added @FlakyTests.
- Other minor improvements, like removing some redundant eq() calls.

Test: atest \
      FrameworksServicesTests:OwnersTest \
      FrameworksServicesTests:FactoryResetProtectionPolicyTest \
      FrameworksServicesTests:DevicePolicyConstantsTest \
      FrameworksServicesTests:DevicePolicyManagerServiceMigrationTest \
      FrameworksServicesTests:NetworkEventTest \
      FrameworksServicesTests:OverlayPackagesProviderTest \
      FrameworksServicesTests:SecurityEventTest \
      FrameworksServicesTests:SystemUpdatePolicyTest \
      FrameworksServicesTests:TransferOwnershipMetadataManagerTest \
      FrameworksServicesTests:DevicePolicyManagerTest

Bug: 171932723
Bug: 30839080

Change-Id: Ie50fa12032aa4c45636ce485d66507ddd4cac16b