Delay for Diagnosis Keys retrieval set to 2 hours (contributes to corona-warn-app/cwa-documentation#236)

[kbobrowski]

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Description

Currently Diagnosis Keys are only fetched every 24 hours, which introduces additional 24 hours delay in exposure notification in a worst-case scenario (last synchronization performed just before relevant Diagnosis Key became available). Decreasing this value to 2 hours also decreases introduced delay to 2 hours in worst-case, and aligns Android implementation with current iOS implementation.

Contributes to corona-warn-app/cwa-documentation#236

[jakobmoellerdev]

Hi! Since Hourly Fetching is not supported yet due to lack of batching as well as the necessity to persist the token for actually combining the results of provideDiagnosisKeys, this PR will not be merged for a while.

If batching gets introduced and the hourly fetching is properly introduced we will instead reintroduce some of the code that was previously implemented but removed before release due to the mentioned lack of batching. Most of that code is equivalent to your PR suggestion.