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ARROW-7437: [Java] ReadChannel#readFully does not set writer index correctly

1. The writer index should be incremented by the amount of data actually read.
2. When EOS is encounterned, the number of bytes read should be incremented before returning.

Closes #6064 from liyafan82/fly_1219_idx and squashes the following commits:

e268b0d31 <liyafan82>  Resolve comments
baab80532 <liyafan82>  ReadChannel#readFully does not set writer index correctly

Authored-by: liyafan82 <fan_li_ya@foxmail.com>
Signed-off-by: David Li <li.davidm96@gmail.com>