The --gtest_color flag should take a priority.

This along with commit c1ef5eb0 should
get back to the initial behavior prior to the change in commit
83b63797.

Fixes: 111396376
Test: `adb shell /data/nativetest64/bionic-unit-tests/bionic-unit-tests --gtest_color=yes`
      shows colored output.
Test: `adb shell -t /data/nativetest64/bionic-unit-tests/bionic-unit-tests --gtest_color=no`
      doesn't add color escapes.
Change-Id: I88530e8b264cd0c259c16b09bdf867cdd2e88224