Do not set the StrongEncryption flag for WinZipAes encrypted entries

[Numpsy]

Change ZipEntry.ProcessAESExtraData to not set the StrongEncryption flag for WinZipAes encrypted entries.

As mentioned in #317, setting the StrongEncryption flag in ProcessAESExtraData doesn't seem correct.
This changes that, and rearranges CreateAndInitDecryptionStream to check for the compression method being WinZipAES rather than for the StrongEncryption flag being set.
This will throw if the archive is really using the PKWare strong encryption, which I think is correct if that isn't actually supported?

Related to this: I'm not sure if the

versionToExtract = ZipConstants.VERSION_AES

in ProcessAESExtraData is correct either (what happens if the existing version is greater than that?). Maybe needs a seperate change?

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[piksel]

This will stop us from writing incorrect headers, whilst still allowing older (incorrect) files to be read as long as they specify the correct CompressionMethod. Nice!