Create a ninja.environment file

This contains the environment variables set by soong_ui during the ninja
execution. This file will be unused in single-tree builds, but will be
useful for multi-tree. The orchestrator will use this file to set the
correct environment for each inner tree build in the combined ninja
execution.

Test: m nothing compiles
Change-Id: I0570e34f51f426448464fb80030d4bea1cd52acb