Selectively add _asan flavor suffix.

This CL ensures that the _asan suffix is added to the build flavor
only when it doesn't already contain _asan (or _asan_coverage). This
correctly prevents it from appending an extra _asan to ASAN lunch
configs that already include it in the flavor, while allowing
sanitized targets for generic configs to behave as they used to.

Bug: 38145756
Test: _asan is correctly appended only for lunch configs that don't
already specify it in the build flavor.

Change-Id: Ia7d9356f717b97d1c64e3237ca31cc507f27734f