Avoid TREBLE_TESTING_OVERRIDE

This is moved from an environmental variable to a function since
getenv is problematic in multi-threaded testing environments.

Bug: 156668058
Test: run each of libhwbinder_benchmark hwbinderThroughputTest libhwbinder_latency
Change-Id: I035b6fbe12536a292e7c473abfb7b135e9400cf0
3 files changed