Final changes for AES GCM encryption streams

[rdblue]

This implements the last few changes for the AES GCM encryption streams (apache#3231).

• Use final variables for reused cipher and plain byte arrays
• Validate that only one partial block can be written by the output stream (the last block)
• Remove unnecessary tracking of streamPosition in the output stream
• Remove some duplication in Ciphers
• Add separate tests to check AAD and nonce validation: bad AAD prefix, corrupt AAD, corrupt nonce, corrupt ciphertext
• Update empty file tests to validate the AAD

The main behavior change is that all files will write at least one cipher block. This is to ensure that the file AAD is validated even for empty files. Otherwise, an attacker could replace a metadata file with an empty file (with a valid header) and nothing would detect it.