DPM: Implement installing certificates for generated keys

Add a new method in the DevicePolicyManager to associate certificates (and
set the user-visibility) with a given key alias.
Conceptually, the new method, setKeyPairCertificate is very similar to
installKeyPair, except it does not install a key, only certificates.

(The new setKeyPairCertificate, together with generateKeyPair is
functionally equivalent to installKeyPair, except the keys are generated
in hardware rather than supplied externally).

Bug: 63388672
Test: cts-tradefed run commandAndExit cts-dev -a armeabi-v7a -m CtsDevicePolicyManagerTestCases -t com.android.cts.devicepolicy.DeviceOwnerTest#testKeyManagement -l DEBUG
Change-Id: Idbfe151f6e5311766decbc1a010bff78dc60249f