Convert art runtests to Android.bp

Build the art runtest libraries with Android.bp.  The host test
libraries move from out/host/linux-x86/lib[64] to
out/host/linux-x86/nativetest[64], and the device test libraires from
/data/artest/${TARGET_ARCH} to /data/nativetest[64]/art/${TARGET_ARCH},
which requires adding nativetest to the library search path, and fixing
some tests to support multiple entries in library search paths.

Test: m -j test-art-host
Test: m -j test-art-target
Change-Id: I2118a3292f22f275954ddd7cdd4e12287fe47808