Fix writedocs.

This probably got broken by aosp/1609473 because soong_build is now
invoked in two different ways:

- By its absolute path during building out/soong/build.ninja
- By its path relative to the source root when generating documentation

Bug: 182347000
Test: Manual (the long incantation on b/182347000)
Change-Id: I0a3a97e4865ffde4a450a3f7baf3980b5a2016c8