MiniThumbFile: Fix image/bitmap retrieval

Change I66005dd69b0a5c8f4353bd7a8225d163a654fd2d changed the insertion
and creation indexing mechanism, but the retrieval method was still
using the old sparse one. Fixit

Fixes android.provider.cts.MediaStore_Images_MediaTest#testInsertImageWithBitmap,
android.provider.cts.MediaStore_Images_MediaTest#testInsertImageWithImagePath,
android.provider.cts.MediaStore_Images_ThumbnailsTest#testQueryExternalMiniThumbnails,
and android.provider.cts.MediaStore_Video_ThumbnailsTest#testGetThumbnail

Change-Id: I41feea79fa304635f3908055fb95a8c85718a97f