Add conversion methods to hidl_string and hidl_vec

Add unit test for libhidl.

Test: mma
Test: make libhidl-test

Bug: 30471989
Bug: 31982575

Change-Id: Ic7d73a10e5cc178786c628e18d3659400b1afb92
diff --git a/Android.bp b/Android.bp
index 4a18dec..f9fbe10 100644
--- a/Android.bp
+++ b/Android.bp
@@ -12,6 +12,10 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+subdirs = [
+    "test",
+]
+
 cc_library_shared {
     name: "libhidl",
     shared_libs: [