PackBitmapIndex: Not buffer inflated bitmap during bitmap creation.

PackBitmapIndex: Not buffer inflated bitmap during bitmap creation.

Currently we're buffering the inflated bitmap entry in
BasePackBitmapIndex to optimize running time. However, this will use
lots of memory during the creation of the pack bitmap index file.

And change 161456, which rewrote the entire getBitmap method, increased
the fetch latency significantly.

This commit introduces getBitmapWithoutCaching method which is used in
the pack bitmap index file creation only and aims to save memory during
garbage collection and not increase fetch latency.

Change-Id: I7b982c9d4e38f5f6193eaa03894e894ba992b33b
Signed-off-by: Yunjie Li <yunjieli@google.com>