Android: Generate JNI code for MediaConstraints

Also improves ownership model by using std::unique_ptr in a couple of
places instead of raw pointers.

Bug: webrtc:8278
Change-Id: I0429ec3c416b5baa1ffa21dad71e0d64b004c446
Reviewed-on: https://webrtc-review.googlesource.com/25020
Commit-Queue: Magnus Jedvert <magjed@webrtc.org>
Reviewed-by: Sami Kalliomäki <sakal@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#20863}

sdk/android/api/org/webrtc/MediaConstraints.java

@@ -28,10 +28,12 @@ public class MediaConstraints {
       this.value = value;
     }
 
+    @CalledByNative("KeyValuePair")
     public String getKey() {
       return key;
     }
 
+    @CalledByNative("KeyValuePair")
     public String getValue() {
       return value;
     }
@@ -83,4 +85,14 @@ public class MediaConstraints {
     return "mandatory: " + stringifyKeyValuePairList(mandatory) + ", optional: "
         + stringifyKeyValuePairList(optional);
   }
+
+  @CalledByNative
+  List<KeyValuePair> getMandatory() {
+    return mandatory;
+  }
+
+  @CalledByNative
+  List<KeyValuePair> getOptional() {
+    return optional;
+  }
 }