mobile/bind/java: fix compiler warnings

Add a missing #include to declare the exported Go function
setContext, and replace old GNU-style struct initializers.

Change-Id: Id1660559236c39505a47368a700c8e0ad834cf6c
Reviewed-on: https://go-review.googlesource.com/24491
Reviewed-by: David Crawshaw <crawshaw@golang.org>

bind/java/seq_android.c.support

@@ -170,7 +170,7 @@ static nstring utf16_decode(jchar *chars, jsize len) {
 		}
 		ndst += encode_rune(buf + ndst, r);
 	}
-	struct nstring res = {chars: buf, len: ndst};
+	struct nstring res = {.chars = buf, .len = ndst};
 	return res;
 }