Change the default dropbox reserve percent from 10% to 0%.

This is needed to stop discarding crashes when the discard is 90% full. There are still other mechanism in dropbox to make sure dropbox does not use more than 10% of the free disk space or more than 10mb.

Change-Id: I2b3857436ddf434b9051296f9d7669dc1fa95a2e
Bug: 260229302
Test: Changing the value has been tested by experiment: https://mendel.corp.google.com/android_platform_settings/studies/SystemServerTelemetryDropboxStoragePublic#details