DatabaseListActivity/Fragment: Correct some copy/paste mistakes.

* app/src/main/res/layout/fragment_database_list: Provide an XML
  layout for the file.
* app/src/main/java/org/glom/app/DatabaseListFragment.java:
  onCreateView(): And use it.
* app/src/main/java/org/glom/app/DatabaseListActivity: Replace
  a use of the table_list with database_list fragment ID.
DatabaseListActivity/Fragment: Rename some API.

Replace mentions of DatabaseIds with SystemIds to match the
terminology used by our GlomContentProvider.
Remove an unused field.
DocumentActivity: Add isCurrentlyLoadingDocument().

So we can avoid using a half-loaded document.
DocumentSingleton: Start with a null Document.

* app/src/main/java/org/glom/app/DocumentActivity.java:
  Do not start with an empty Document. A null makes it more obvious
  when we have not even started loading it yet, even if we need
  more checks then.
* app/src/main/java/org/glom/app/*Activity.java: Check for the null.
In progress.
GlomContentProvider: Added comments about the SQL tables.
Document.getTablePrimaryKeyField(): Add a null check.
Pass the system_id to Activities and Fragments.
GlomContentProvider: Provide db rows via system/123/table/sometablename.

Instead of using our GlomCursorLoader. Using a ContentProvider lets
us do things the way that the android API seems to prefer, and will
let us use some other features that also expect a ContentProvider.
However, there is some duplication of Document data in the two
processes now.
TablListActvity: getTableListFragment(): Correct the fragment ID.

So we can really find the fragment and really tell it to update.
TableDetailActivity/Fragment: update() it when the document has loaded.

Do not try to build the UI before the document has fully loaded.