Add an example app for Android native API.

The app is a simple loopback demo demonstrating the usage of Android
native API. This is an initial version and I will add support for
HW codecs etc. in the future.

Bug: webrtc:8769
Change-Id: Ifb6209769dabeb8ca3185b969a1ef8afd6d84390
Reviewed-on: https://webrtc-review.googlesource.com/60540
Reviewed-by: Magnus Jedvert <magjed@webrtc.org>
Commit-Queue: Sami Kalliomäki <sakal@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#22385}

sdk/android/native_api/jni/java_types.cc

@@ -266,6 +266,17 @@ void JavaMapBuilder::put(const JavaRef<jobject>& key,
   JNI_Map::Java_Map_put(env_, j_map_, key, value);
 }
 
+jlong NativeToJavaPointer(void* ptr) {
+  static_assert(sizeof(intptr_t) <= sizeof(jlong),
+                "Time to rethink the use of jlongs");
+  // Going through intptr_t to be obvious about the definedness of the
+  // conversion from pointer to integral type.  intptr_t to jlong is a standard
+  // widening by the static_assert above.
+  jlong ret = reinterpret_cast<intptr_t>(ptr);
+  RTC_DCHECK(reinterpret_cast<void*>(ret) == ptr);
+  return ret;
+}
+
 // Given a list of jstrings, reinterprets it to a new vector of native strings.
 std::vector<std::string> JavaToStdVectorStrings(JNIEnv* jni,
                                                 const JavaRef<jobject>& list) {

sdk/android/native_api/jni/java_types.h

@@ -285,6 +285,11 @@ ScopedJavaLocalRef<jobject> NativeToJavaMap(JNIEnv* env,
   return builder.GetJavaMap();
 }
 
+// Return a |jlong| that will correctly convert back to |ptr|.  This is needed
+// because the alternative (of silently passing a 32-bit pointer to a vararg
+// function expecting a 64-bit param) picks up garbage in the high 32 bits.
+jlong NativeToJavaPointer(void* ptr);
+
 // ------------------------
 // -- Deprecated methods --
 // ------------------------