DfsReader/PackFile: Implement isNotLargerThan using the obj size idx

DfsReader/PackFile: Implement isNotLargerThan using the obj size idx

isNotLargerThan() can avoid reading the size of a blob from disk using
the object size idx if available.

Load the object size index in the DfsPackfile following the same
pattern than the other indices. Override isNotLargerThan in DfsReader
to use the index when available.

Following CL introduces the writing of the object size index and the
tests cover this code.

Change-Id: I15c95b84c1424707c487a7d29c5c46b1a9d0ceba