PackBitmapIndex: Not buffer inflated bitmap in BasePackBitmapIndex

PackBitmapIndex: Not buffer inflated bitmap in BasePackBitmapIndex

Currently we're buffering the inflated bitmap entry in BasePackBitmapIndex
to optimize running time. However, this will use lots of memory during
the construction of the pack bitmap index file which may cause failure of
garbage collection.

The running time didn't increase significantly, if there's any increase,
after removing the buffering here. The report about usage of time/memory
will come in the next commit.

Change-Id: I874503ecc85714acab7ca62a6a7968c2dc0b56b3
Signed-off-by: Yunjie Li <yunjieli@google.com>