Update DfsObjDatabase API to open/write by pack extension.

Update DfsObjDatabase API to open/write by pack extension.

Previously, the DfsObjDatabase had a hardcoded getPackFile() and
getPackIndex() methods which opens a .pack and .idx file, respectively.
A future change to add a bitmap index will need to be stored in a
parallel .bitmap file. Update the DfsObjDatabase to support opening and
writing of files for any pack extension.

Change-Id: I7c403b501e242096a2d435f6865d6025a9f86108