Rework usage of piwik

[jklmnn]

See #45 As we want to keep the users privacy, it is required to keep the data as small as possible.

What we might want to know:

• what city is used (or is automatically used)
• Where the app was installed from (F-Droid, local or PlayStore, automatically gathered by Piwik)
• Probably what activity is used.

What definitely shall not be gathered:

• exact location (used city is sufficiently exact)
• search terms as they might contain names or addresses.

[jklmnn]

Removed Piwik completely due to complexity and privacy (https://github.com/jklmnn/ParkenDD/commit/8be563a09dde708fa28cd05b7c248760a55952a9).