Add flag to skip writing output from soong_zip.

We want to measure how much of build time is spent on writing large
zips to disk. We can do this by doing the rest of the work but
skipping the write itself.

Some local and trivial testing shows this is probably a lot of the
cost of these rules:

    $ python -m timeit -s "import os" -r 10 "os.system('soong_zip -write_if_changed -o foo.zip -D prebuilts/sdk')"
    1 loop, best of 10: 8.37 sec per loop
    $ python -m timeit -s "import os" -r 10 "os.system('soong_zip -o foo.zip -D prebuilts/sdk')"
    1 loop, best of 10: 5.39 sec per loop
    $ python -m timeit -s "import os" -r 10 "os.system('soong_zip -n -o foo.zip -D prebuilts/sdk')"
    1 loop, best of 10: 3.66 sec per loop

Bug: None
Test: treehugger
Change-Id: I43acdb08d0e00efaba9e5d7add972a7ec41646d2