Prototype Content Provider support for MTP/PTP devices.

At this point much of the plumbing is in place, but only a few simple queries
are supported.
This is enough to support a proof of concept sample program that navigates
the file hierarchy of a digital camera connected via USB.

Also removed obsolete ptptest host test program.

Change-Id: I17644344b9f0ce1ecc302bc0478c1f3d44a1647f
Signed-off-by: Mike Lockwood <lockwood@android.com>