Add ASAN_ENABLED and SANITIZE_TARGET_SYSTEM_ENABLED to Soong config variables:

`SANITIZE_TARGET` is a variable in Android's build system used to
enable sanitizers. Currently, Soong only supports enabling SANITIZE for
individual modules in Android.bp.
Adding ASAN_ENABLED and SANITIZE_TARGET_SYSTEM_ENABLED as Soong config
variables would allow Soong to also support building with Sanitizers by
directly setting the SANITIZE_TARGET environment variable.

Bug: 353164536
Test: SANITIZE_TARGET_SYSTEM=true SANITIZE_TARGET=address m -j asan_extract
Change-Id: I542239ee25d210d903d2481a2b1426f756656308