toolcontext: don't fallback to the env variable

* we'd rather specify this directly, android's init can be
  a bit finicky about environment variables and when they get used
  and where they're available

* hardcode it

Change-Id: I0df853a173c1757ec908e154055f816deba9202d
README.md: add android instructions

Change-Id: Idf1c2f6ee12841648d7d77ffcfe29a06666040d8