Fix race condition writing soong.config

soong_build is run twice simultaneously now, once for manifest
generation and once for docs generation.  If one starts writing the
default soong.config file, and the other starts reading it, the reader
can see an empty file and fail.  Write the soong.config file to a
temporary file and the atomically rename it into place.

Bug: 32628314
Test: rm out/soong/soong.config && m -j blah && cat out/soong/soong.config
Change-Id: I8119b11d45093284b24cbc926d81eb9ea4bf2e27