[FlexBuffers][Java] Fix wrong access to a string using Reference::asString(). (#5532)

The real position of a string is  calculated by using the indirect() method,
which should be based on parentWidth and not byteWidth, as it was implemented.

We are also fixing the flag BUILDER_FLAG_SHARE_STRINGS on FlexBuffersBuilder
that was set as '1', same value as BUILDER_FLAG_SHARE_KEYS.

java/com/google/flatbuffers/FlexBuffers.java

@@ -481,7 +481,7 @@ public class FlexBuffers {
          */
         public String asString() {
             if (isString()) {
-                int start = indirect(bb, end, byteWidth);
+                int start = indirect(bb, end, parentWidth);
                 int size = readInt(bb, start - byteWidth, byteWidth);
                 return Utf8.getDefault().decodeUtf8(bb, start, size);
             }