Make test_suites arch_variant

In most cases, we don't care about arm vs x86 when selecting whether a
module is in a test suite or not, but we do care about whether we need
to add the host or device module(s) to the suite. So enable the arch
mutator.

Test: Add android-specific test_suites property, look in out/soong/Android.mk
Change-Id: I1e258d6e97e18c8d20297568cd8ee8d4161d5023