Use hashed subdir for soong_config modules

This is to differentiate soong intermediate directories for soong config
modules. This will help incremental build across different
devices.

Test result of building panther, building cheetah, and building panther
again:

Before this change
- build time: 02:57
- # of tasks: 31044

After this change
- build time: 01:48
- # of tasks: 1694

Build time includes build.ninja generating time (which is more than 1
minute), so the overriden artifacts become far fewer.

And "NINJA_ARGS='-n -d explain' m" only gave 4 "command line changed"
nodes.

Bug: 279362051
Test: see above
Change-Id: I4891cbe823ae21628465e5c6eb26a4837ccdd202