Add KeyChain Test API for the credential manager

(Cut down version)
* Add permission to manage credential
  management app, which is to be used in
  CTS tests

Bug: 179180345
Test: atest android.devicepolicy.cts.CredentialManagementAppTest
Change-Id: I8487ebc13758a31639d55c8e380faa51d1cfd843
Merged-In: I8487ebc13758a31639d55c8e380faa51d1cfd843