Make PackWriterBitmapWalker public

Make PackWriterBitmapWalker public

Make PackWriterBitmapWriter class public and move it to a more central
location, in preparation for its use by another class (in a subsequent
commit).

One of its inner static classes, AddUnseenToBitmapFilter, previously
package-private, is also used directly in its former package. Therefore,
AddUnseenToBitmapFilter and its sibling class have been moved to an
internal package instead.

Change-Id: I740bc4bfc4e4e3c857d1ee7d25fe45e90cd22a75
Signed-off-by: Jonathan Tan <jonathantanmy@google.com>