Add DPM.getKeyPairGrants()

(Trimmed down version)

Bug: 179180345
Test: atest
com.android.cts.devicepolicy.MixedManagedProfileOwnerTest#testKeyManagement
Test: atest
com.android.cts.devicepolicy.MixedDeviceOwnerTest#testDelegatedCertInstallerDirectly
Test: atest android.admin.cts.DevicePolicyManagerTest
Change-Id: Ieee1048b145fb8400eccbf32c054afc64b5d90c0
Merged-In: Ieee1048b145fb8400eccbf32c054afc64b5d90c0

Change-Id: Id1ab42ddbf04a3f6874ba5ce7eb937114d2194ef