On uninstall cred mng app

Changes
* when ACTION_PACKAGE_REMOVE is received
  in the DPMS broadcast receiver, remove
  the cred mng app if the removed package
  is the cred mng app.

Manual testing steps:
* Request to manage credentials in TestDPC
* Check Settings > encryption & credentials
  and verify the cred mng app is set successfully
* Uninstall TestDPC
* Check Settings to and verify there is no
  cred mng app.

Bug: 165641221
Test: Manual testing
      atest com.android.server.devicepolicy.DevicePolicyManagerTest
Change-Id: I6e8004da40c35c0dfe25540bf143f255d9f839d7