Add ART gtests to the Mainline Test Suite (MTS).

Include the `ArtGtestsTargetInstallApex` module in the MTS test suite.

Also support multilib variants in this Soong module (using different
suffix per sub-architecture), which is needed on build targets with
secondary architectures, as the MTS test suite packaging logic
flattens all test artifacts into a single `testcases` directory.

Test: Run `m mts` and check the contents of the MTS package
      (`out/host/linux-x86/mts/android-mts.zip`)
Bug: 167385698
Bug: 168984433
Change-Id: I123f313db5ab32b38c2110301bb84eb208a58047
diff --git a/test/Android.bp b/test/Android.bp
index b3889f3..d31de87 100644
--- a/test/Android.bp
+++ b/test/Android.bp
@@ -984,8 +984,24 @@
         ":com.android.art.testing",
     ],
     // It seems that only device-tests build can create a valid apex.
-    test_suites: ["device-tests"],
+    test_suites: [
+        "device-tests",
+        "mts",
+    ],
     test_config: "art-gtests-target-install-apex.xml",
+
+    // Support multilib variants (using different suffix per sub-architecture), which is needed on
+    // build targets with secondary architectures, as the MTS test suite packaging logic flattens
+    // all test artifacts into a single `testcases` directory.
+    compile_multilib: "both",
+    multilib: {
+        lib32: {
+            suffix: "32",
+        },
+        lib64: {
+            suffix: "64",
+        },
+    },
 }
 
 art_cc_test {