Automate dependency on inputs of genrule module type.

This change automates one of the best practices for
custom tools. The best practice says "Prefer to list
input files on the command line, otherwise we may not
know to re-run your command when a new input file is
added." [1]

Normally you'd reference these inputs with $(in) or
one of the forms of the $(location*) substituions on
the command line for the custom tool. However, if the
custom tool does not accept the list of inputs on the
command line, the build system could fail to re-run
the custom tool if the list changes.

This change adds a hash of the list of input names to
the sbox command that wraps the custom tool. If the
list of inputs change, the hash will change, and
therefore the sbox command will change, causing ninja
to re-run the custom tool. The hash is visible to
(but ignored by) the sbox command, and hidden from
your custom tool.

[1] https://android.googlesource.com/platform/build/soong/+/refs/heads/master/docs/best_practices.md#custom-build-tools

Test: TestGenruleHashInputs
Bug: 149397658
Change-Id: I18b547ea3c4296ee15bd6150a4778a8f376d80b7