Make isolated true by default for gtest

This change sets everything up to do this, but does not actually
enable it. jdesprez will follow up by making a one line change
that actually turns the default to true when he does test
debugging/verification.

Bug: 178498003
Test: build. Relying on @jdesprez for the rest of the verification
Change-Id: I3b1b6b57279dd4f9f7fc559e2d3ad76911b045e8