Java: Calculation of vtable and vtable size moved to the __init method. (#5210)

vtable and vtable size depends only on `Table#bb_pos` but calculated in
`Table#_offset` method on each field lookup.
Doing this with every call of `Table#__offset` is redundant.

These values can be read once with change of `Table#bb_pos` and reused
for any field lookup.