Specify primary mime type for each format code.

MediaFile#getMimeTypeForFormatCode returns mimetype for MTP format
code. However MTP format code is ambiguous and some format code can be
mapped to several mime types.  Before the fix, the method returns mime
type that is last registered to internal static map.

The CL explicitly specifies the primary mime type for each format code
to prevent from unintentionally changing mime type returned for a format
code.

FIX=30266194

Change-Id: I142ccb9719d439b0378121bcdcd4b804b1505952