Add var to enable/disable tethering next APEX

This is necessary to disable the .inprocess APEX, in branches that only
use stable APIs, as it depends on unstable stubs. override_apex does not
support defaults.

Bug: 190249673
Test: m
Change-Id: Ib4a27b65f3c5b6d4b0ac69e85cf671038efc5744
1 file changed