- This is part of the work to support
  a credential management app on
  unmanaged devices.
- Add intent and method in KeyChain to allow
  an app to request to become the credential
  management app.
- Add the class CredentialManagementApp to store the
  current credential management app.
- Add the class AppUriAuthenticationPolicy and an
  extra in KeyChain to allow an app to set an
  authentication policy.
- Add API methods to KeyChainService to set, get
  and retrieve the credential management app.

Bug: 165641221
Test: atest CredentialManagementAppTest
      atest AppUriAuthenticationPolicyTest
      adb shell am start -n com.android.keychain.tests/.KeyChainTestActivity
Change-Id: I1e57ed9c18a1ada463c55dbf17ce30e31aa7bad2