setKeyGrantForApp DELEGATION_CERT_SELECTION

* setKeyGrantForApp and setKeyGrantToWifiAuth
  should be guarded with the
  DELEGATION_CERT_SELECTION instead of
  DELEGATION_CERT_INSTALL
* Update DPM docs

Bug: 178598508
Test: atest com.android.server.devicepolicy.DevicePolicyManagerTest
      atest com.android.cts.devicepolicy.MixedDeviceOwnerTest#testDelegation
      atest com.android.cts.devicepolicy.MixedDeviceOwnerTest#testDelegationCertSelection
      atest com.android.cts.devicepolicy.MixedDeviceOwnerTest#testDelegatedCertInstallerDirectly

Change-Id: Ia47c1e1b360d8e60d6a2e56a7b449f3113d40352