Match DPM key grant API changes

Since the single DevicePolicyManager method for setting/revoking key
grants was split into two, grantKeyPairToApp revokeKeyPairFromApp,
modify the tests to use the new API.

Bug: 139091816
Test: cts-tradefed run commandAndExit cts-dev -m CtsDevicePolicyManagerTestCases -t com.android.cts.devicepolicy.MixedDeviceOwnerTest#testSetKeyGrant
Change-Id: I1e9701670bcdafcb756e862c138239a35f959aaa