Show loading, error, and info messages as footers.

A provider can include extras in their Cursors to indicate that
loading is ongoing, or include an error or informational message,
which are now shown in footer views.

Fix registration to always get change notifications.

Test provider that verifies common provider behavior of holding
a reference to "cloud" resources that are released by GC when the
remote Cursor is closed.  Also used to validate Recents behavior
for slow providers.

Bug: 10599268
Change-Id: I331c31058dbb80261e7d279b851197c65ac87e32