Open archives in different modes separately.

If an archive is opened for writing, and then for reading, we
have to keep both opened. Until now, we'd only open one, and reuse
the first instance for both, causing exceptions.

Test: Tested manually.
Bug: 20822019
Change-Id: I28eb2a67ca43ecc2d35fa9cfae4e20200e8a46bc
(cherry picked from commit 777efaea)