gomobile Reader.Read should take offset and length params

[kaloyan-raev]

What: Reader.Read in gomobile accepts offset and length params.

In addition, this PR fixes a bug in the usage of length in Writer.Write method.

Why: This allows more optimal usage from Java code. Java does not have slices like in Golang, but achieves the same on byte arrays with the usage of offset and length.

Please describe the tests:

• Existing tests adapted

Please describe the performance impact:

Code Review Checklist (to be filled out by reviewer)

• Does the PR describe what changes are being made?
• Does the PR describe why the changes are being made?
• Does the code follow our style guide?
• Does the code follow our testing guide?
• Is the PR appropriately sized? (If it could be broken into smaller PRs it should be)
• Does the new code have enough tests? (every PR should have tests or justification otherwise. Bug-fix PRs especially)
• Does the new code have enough documentation that answers "how do I use it?" and "what does it do?"? (both source documentation and higher level, diagrams?)
• Does any documentation need updating?
• Do the database access patterns make sense?