[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.