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

mime/java-res/android.mime.types

@@ -139,7 +139,7 @@
 # that may have been defined earlier.
 
 application/pgp-signature pgp
-application/x-x509-ca-cert crt
+application/x-x509-ca-cert crt der
 audio/aac aac adts adt
 audio/basic snd
 audio/flac flac