DatabaseListActivity: Slight code rearrangement.
AndroidManifest.xml: Open URLs in the main activity instead of TableNavActivity.

Otherwise, users will be offered the "Tables" activity instead of the
"Android Glom" activity, which won't make sense.
DatabaseListActivity: Let this open URIs.

* app/src/main/java/org/glom/DocumentActivity.java: Move the document-loading
  code into a GlomActivity superclass.
* app/src/main/java/org/glom/DatabaseListActivity: Derive this from
  GlomActivity too, so it can open files.
  onDocumentLoadingExampleFinished(): start it in a TableNavActivity
  via the new systemId.