Add DPM.hasKeyPair()

The method returns true if a private key and a user certifiate are
present in KeyChain under this alias.

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: I15ca6190db1ef4dbf0caabe9d46008d92f758df5