2.21.0-beta5 should respect the background periods properly

[elican-doenyas]

Currently, when the library user configures scan periods for background scanning, the library discards the backgroundBetweenScanPeriod, and uses the backgroundScanPeriod for that instead. This is present in 2 places in BeaconManager class, both in the applySettingsChange method.

1. Line 258

2. Line 275

Also, while I am not sure of this, the default value for backgroundBetweenScanPeriod in Settings.kt file, ScanPeriods data class should not be 0.

@davidgyoung thanks in advance.