PackBitmapIndexBuilder: allow repeated call of getCompressedBitmaps()

PackBitmapIndexBuilder: allow repeated call of getCompressedBitmaps()

Each call of the current implementation of
PackBitmapIndexBuilder.getCompressedBitmaps reverses the class member that is returned. So two subsequent calls of this function will
receive reversed structures.

Instead we should just reverse a copy of the structure. This way the
PackBitmapIndexBuilder can continue to efficiently accept new bitmaps
via processBitmapForWrite (because we can continue to add to the back of
the bitmapsToWrite ArrayList) while ensuring that the bitmaps are
returned in the correct ordering from getCompressedBitmaps.

Change-Id: If7364222b9e62920c0c2c06894716fe85dd13315