mobile: update measurement-log retention policy

[aanorbel]

Describe the bug

App makes multiple calls to the database and check_report_id API.

To Reproduce

Run the app with hundreds of measurements and watch debug logs.
App takes a lot of time to process each measurement and determine whether to delete logs

Expected behaviour

The process should take the shortest possible time.

System information (please complete the following information):

• Device: all
• OS: all
• OONI Probe version: all

Additional context

Add any other context about the problem here.