Add CTS tests for DPM.getKeyPairGrants()

Bug: 160457441
Test: atest com.android.cts.devicepolicy.MixedManagedProfileOwnerTest#testKeyManagement
Test: atest com.android.cts.devicepolicy.MixedDeviceOwnerTest#testDelegatedCertInstallerDirectly
Test: atest android.admin.cts.DevicePolicyManagerTest
Change-Id: If47cb7774993647d056b97aa9cf200a6c9f2419a
7 files changed