convert internal dict keys to match fdroidclient's Apk class

convert internal dict keys to match fdroidclient's Apk class

This syncs up the field names between the fdroiddata .yml files, the keys
used in the implementation in fdroidserver, the index data format, and the
final data structures in fdroidclient.  This makes it easier for devs to
follow, and makes the Jackson parsing library automatically handle
converting the data from the index file to Java instances.

This bumps the metadata version since the apkcache will have to be
discarded.

Here are the name changes:
* apkname --> apkName
* id --> packageName
* sha256 --> hash
* version --> versionName
* versioncode --> versionCode

tests/repo/index.xml was changed only to bump the metadata version
from 17 to 18.