releasetools: Allow generating BBOTA for images with shared blocks.

When target defines 'BOARD_EXT4_SHARE_DUP_BLOCKS := true', the generated
system/vendor images may contain shared blocks (i.e. some blocks will
show up in multiple files' block list), which violates the current
assumptions in BBOTA script.

This CL allows generating BBOTAs by considering the first occurrence as
the "owner" of the shared blocks. All the later users of the shared
blocks will have an incomplete block list, whose RangeSet's will be
tagged with 'uses_shared_blocks'.

Files with 'uses_shared_blocks' tag will not be diff'd with imgdiff,
potentially with patch size penalty. Such files will be accounted for in
imgdiff stats report, where we can revisit for a better solution.

Bug: 64109868
Test: Generate BBOTA full and incremental package with targets defining
      'BOARD_EXT4_SHARE_DUP_BLOCKS := true'.
Change-Id: I87fbc22eef7fafe2a470a03fdcfa1babf088ea8d