Better status update

Keyguard prompt is part of the installation flow.
We should post a status update to notify the caller if
the keyguard is dismissed and installation won't continue.

Bug: 277691885
Test: 1. set up lock screen password
      2. kick off DSU installation
      3. press back key to dismiss keyguard
      4. check logcat messages and find
         "DynamicSystemInstallationService: status: NOT_STARTED, cause: INSTALL_CANCELLED"
Change-Id: I0c919841d756ada6e94414e58f310635e4d8c12c