[Java][FlexBuffers] Abstract buffer access from ByteBuffer (#5743)

To read and build flexbuffers on Java, one needs to wrap the data
using  ByteBuffer. But for the common case of having ByteBuffers
backed by arrays, accessing from a ByteBuffer might be inefficient.

So this change introduces two interfaces: ReadBuf and ReadWriteBuf.
It allows one to read and writes data directly on an array. It also allow
 other buffer implementations to be used with flexbuffers.

Another change is that FlexBuffersBuilder backed by array allows
the buffer to grow with the increase of the message size. Something
that could not be done with ByteBuffer.

java/com/google/flatbuffers/ByteBufferReadWriteBuf.java

@@ -0,0 +1,165 @@
+package com.google.flatbuffers;
+
+import java.nio.ByteBuffer;
+import java.nio.ByteOrder;
+
+public class ByteBufferReadWriteBuf implements ReadWriteBuf {
+
+  private final ByteBuffer buffer;
+
+  public ByteBufferReadWriteBuf(ByteBuffer bb) {
+    this.buffer = bb;
+    this.buffer.order(ByteOrder.LITTLE_ENDIAN);
+  }
+
+  @Override
+  public boolean getBoolean(int index) {
+    return get(index) != 0;
+  }
+
+  @Override
+  public byte get(int index) {
+    return buffer.get(index);
+  }
+
+  @Override
+  public short getShort(int index) {
+    return buffer.getShort(index);
+  }
+
+  @Override
+  public int getInt(int index) {
+    return buffer.getInt(index);
+  }
+
+  @Override
+  public long getLong(int index) {
+    return buffer.getLong(index);
+  }
+
+  @Override
+  public float getFloat(int index) {
+    return buffer.getFloat(index);
+  }
+
+  @Override
+  public double getDouble(int index) {
+    return buffer.getDouble(index);
+  }
+
+  @Override
+  public String getString(int start, int size) {
+    return Utf8Safe.decodeUtf8Buffer(buffer, start, size);
+  }
+
+  @Override
+  public byte[] data() {
+    return buffer.array();
+  }
+
+  @Override
+  public void putBoolean(boolean value) {
+    buffer.put(value ? (byte)1 : (byte)0);
+  }
+
+  @Override
+  public void put(byte[] value, int start, int length) {
+    buffer.put(value, start, length);
+  }
+
+  @Override
+  public void put(byte value) {
+    buffer.put(value);
+  }
+
+  @Override
+  public void putShort(short value) {
+    buffer.putShort(value);
+  }
+
+  @Override
+  public void putInt(int value) {
+    buffer.putInt(value);
+  }
+
+  @Override
+  public void putLong(long value) {
+    buffer.putLong(value);
+  }
+
+  @Override
+  public void putFloat(float value) {
+    buffer.putFloat(value);
+  }
+
+  @Override
+  public void putDouble(double value) {
+    buffer.putDouble(value);
+  }
+
+  @Override
+  public void setBoolean(int index, boolean value) {
+    set(index, value ? (byte)1 : (byte)0);
+  }
+
+  @Override
+  public void set(int index, byte value) {
+    requestCapacity(index + 1);
+    buffer.put(index, value);
+  }
+
+  @Override
+  public void set(int index, byte[] value, int start, int length) {
+    requestCapacity(index + (length - start));
+    int curPos = buffer.position();
+    buffer.position(index);
+    buffer.put(value, start, length);
+    buffer.position(curPos);
+  }
+
+  @Override
+  public void setShort(int index, short value) {
+    requestCapacity(index + 2);
+    buffer.putShort(index, value);
+  }
+
+  @Override
+  public void setInt(int index, int value) {
+    requestCapacity(index + 4);
+    buffer.putInt(index, value);
+  }
+
+  @Override
+  public void setLong(int index, long value) {
+    requestCapacity(index + 8);
+    buffer.putLong(index, value);
+  }
+
+  @Override
+  public void setFloat(int index, float value) {
+    requestCapacity(index + 4);
+    buffer.putFloat(index, value);
+  }
+
+  @Override
+  public void setDouble(int index, double value) {
+    requestCapacity(index + 8);
+    buffer.putDouble(index, value);
+  }
+
+  @Override
+  public int writePosition() {
+    return buffer.position();
+  }
+
+  @Override
+  public int limit() {
+    return buffer.limit();
+  }
+
+  @Override
+  public boolean requestCapacity(int capacity) {
+    return capacity <= buffer.limit();
+  }
+
+}