DPM key grant: Address API review

Split the single method for granting/revoking key access into two
methods.

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

Change-Id: I495f35419a5dd1c12e18703f11dd4216a310430d