Fix --dalvik-cache

If we passed "--dalvik-cache=foo", then the artifact directory was
being set to /data/misc/apexdata/com.android.art/foo/foo instead of
/data/misc/apexdata/com.android.art/foo. This actually only affects
the cache-info.xml which I think is why I didn't notice before.

While I'm here, updated & tidied the usage info a bit.

Bug: 200020887
Test: composd_cmd forced-odrefresh
Change-Id: I6c74a54ebb4cd93506f0184016d6e1b04052db85
1 file changed