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: atest VtsVndkHidlBufferpoolV1_0TargetMultiTest VtsVndkHidlBufferpoolV2_0TargetMultiTest
Change-Id: I6e6b48796686098d63f09d965c3252140352d0d9