setTrebleTestingOverride

This is being added in preparation of removing TREBLE_TESTING_OVERRIDE
since some clients may use setenv/etc in a multithreaded environment
which break the use of getenv to retrieve the value of this flag.

Bug: 156668058
Test: tests using either flag still work
Change-Id: I53e3fafa373e19814d9533ade0587e32278dfec7
2 files changed