Add new DevicePolicyConstant for indicating that the testOnly admin should be...

Add new DevicePolicyConstant for indicating that the testOnly admin should be used as the supervision component.

We were always using the testOnly admin previously, but this shouldn't apply for all test cases, thus making it conditional now.

Short term fix until general test solution to override xml resources (which is how the designated supervision component is set) is implemented (b/182994391).

Bug: 176212263
Test: atest FrameworksServicesTests:DevicePolicyManagerTest
Test: atest -c 'CtsDevicePolicyManagerTestCases:com.android.cts.devicepolicy.MixedProfileOwnerTest'
Change-Id: I81506362ea681ca01d98d1713220110977f0fee1