Enable reading a thumbnail from RAW image files in MtpDatabase

Also introduce new supported RAW image file formats, PEF and SRW.

RAW image file formats are not defined in PTP 1.2 specification except
for DNG. They are mostly built on top of TIFF or TIFF/EP. (Fuji's RAF
is the exception).

In this CL, image file formats are classified newly as below:

DNG: dng
TIFF: cr2, nrw, arw, rw2, orf, pef, srw
TIFF/EP: nef
Unknown Image Formats(FORMAT_DEFINED): wbmap, webp, raf

I referred to the following documents for defining MTP formats of RAW
images:

* http://www.rags-int-inc.com/PhotoTechStuff/RawStandards/RawSummary.html
* https://en.wikipedia.org/wiki/Raw_image_format

Bug: 26552863, Bug: 26626825
Change-Id: Ia218f6320c4c1ff051a23ca0060ceac46134b0d7