Impure incremental builds by allowing the intermediate products dir in the sandbox

[Atemu]

Pure incremental builds are quite complex to support but for the time being, perhaps one could make use of the AOSP build system's own internal build caching by allowing its directories in the sandbox.

Obviously this isn't great and very impure but I trust the build system enough to get incremental builds at least correct (even if they might not be fully reproducible) and that's good enough for hacking.