Made FlatBufferBuilder.java require ByteBuffers that have an array.

Readonly ByteBuffers and Direct ByteBuffers don't have a backing
array, and thus can't be used for writing FlatBuffers (though
they are fine for reading).

Change-Id: I4d7b9cc222b96161d0f8e92f34588bd3e0e38034
Tested: on Linux.