Add a test app: quota exceeded doing a backup

This is a simple test app (a clone of the Backup/Restore sample code)
that deliberately backs up 20 megabytes of data every time it performs
a backup pass.  This is to trigger the quota-exceeded logic both on
the client and on the backend, for testing purposes.

Change-Id: I01f04d0f85c827baadf1990c09643d5eb5f88266