Allow specifying sub-dir in sbox output

xsdc commands end up using the same output directory inside sbox
for reading and writing the output archive, resulting in including
a partial copy of the archive in itself.

This change allows that rule to specify a sub-directory for the inputs
leaving the output archive outside of it.

Used by https://r.android.com/2955427

Bug: b/322788229
Test: Ran a couple of builds and confirmed no cache misses from xsdc.
Also manually checked an archive and confirmed
abcence of partial self-copies.

Change-Id: If709b111458a83d58906d919c3d2a7787bcbe4ee