Start of restructuring to allow overlays include subset of providers

This allows us to build a UnifiedEmail that supports all providers
and use this base to build applications that only supports a subset
of the providers

Made AccountCacheProvider and ConversationProvider abstract.  This allows
the extending class to specify the authority that should be used (This allows
both UnifiedEmail and Gmail to be installed at the same time)

Change-Id: I31295f4630906e7182e423fcbb7b47c33ba4cd4f
diff --git a/unified_src/README b/unified_src/README
new file mode 100644
index 0000000..ea96076
--- /dev/null
+++ b/unified_src/README
@@ -0,0 +1,9 @@
+This is a temporary directory that is being used to enable email and Gmail content to be shown
+in the UnifiedEmail application
+
+This code should only be used for development.  For shipping, this code (or something similar)
+should go in the Gmail/Email overlay of UnifiedEmail.  But for now, this allows us to use a single
+application to test multiple content sources
+
+This code is just a shim to register existing accounts (Gmail and Email) from the device with the
+AccountCacheProvider.  All subsequent uris will actually be handled by the appropriate application.