Allow installation of cert with .der extension

* Add der file extension to
  application/x-x509-ca-cert MIME type.

Manual testing steps
* Push .der certificate onto device
* Install CA certificate via Settings
  and select .der certificate
* Verify certificate has been
  successfully installed

Bug: 185283683
Test: manual testing
Change-Id: I0be1897a0aaf6f771c0e534533d2413ff0fc12f6