Add KeyChainService credential management app APIs

Background
* This is part of the work to support
  a credential management app on
  unmanaged devices.
Changes
* Add KeyChainService API methods to
  modify the stored credential management app.

Manual Testing
* Install TestDPC
* Request to manage credentials (fire intent).
  Add policy mapping: 'com.android.chrome' ->
  'client.badssl.com:443' -> 'testAlias'
* Install badssl user certificate as credential
  management app (TestDPC). Set alias to 'testAlias'
* Check certificate is installed in Settings
* Go to chrome > client.badssl.com
* Verify no certificate selection prompt is
  displayed. User is automatically authenticated.
* Remove credential management app from Settings
  Security > Encryption and credentials >
  Certificate management app
* Verify credential management app is removed and
  'testAlias' is uninstalled.

Bug: 165641221
Test: Manual Testing
      atest com.android.keychain.KeyChainServiceRoboTest

Change-Id: I00b7df27a92f6ee4f74546f892c83290fead1112