Fallback to the previous weather location in background

Description

Fallback to the previous weather location in background so that the weather updates are done in background.

Small other fixes.

Screenshots

Technical details

When a service is in background, location is only updated a few times every hour (https://developer.android.com/about/versions/oreo/background-location-limits).

Indeed, when the weather service was in background, the location returned was always null.

We now fallback to the previous weather location.

Tests

Issues

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