Both build 32 and 64 architecture for TetheringTests

Compile as multilib is neccessary, otherwise mts-tethering-coverage
test plan will fail on armeabi-v7a because of failing to extract
native libraries. This change also add mts tag to TetheringTests.

Bug: 145490751
Test: -atest TetheringTests
      -m mts && mts-tradefed run mts-tethering-coverage

Change-Id: I6425c135ca17213bcdd5198177993daf8280504e

packages/Tethering/tests/unit/Android.bp

@@ -20,7 +20,11 @@ android_test {
     srcs: [
         "src/**/*.java",
     ],
-    test_suites: ["device-tests"],
+    test_suites: [
+        "device-tests",
+        "mts",
+    ],
+    compile_multilib: "both",
     static_libs: [
         "androidx.test.rules",
         "frameworks-base-testutils",