Load RBE related env vars from config files

This is as part of an effort to move environment variables into config
files so that configs can be tied down to source.

Test:
Ran a build with "ANDROID_BUILD_ENVIRONMENT_CONFIG=googler m"  which ran
a non-RBE build (since this is vanilla aosp).
Also ran a build with "ANDROID_BUILD_ENVIRONMENT_CONFIG= m" which ran a
non-RBE build.

https://paste.googleplex.com/5637282880028672

Bug: b/194679562
Change-Id: I416e8da75f84aa2b53995f525cf50501488dc972