Migration to Android 12

[philips77]

This PR raises the targetSdkVersion to 31 and fixes issues caused by that:

• Adds PendingIntent.MUTABLE flag to PendingIntent. This is required for scanning to work, otherwise the results cannot be delivered. (Scanner broken when targeting Android 12+ #106)
• Sets maxSdkVersion to 30 for BLUETOOTH and BLUETOOT_ADMIN permissions.
• Adds new BLUETOOT_SCAN permission (more).
• Removes ACCESS_FINE_LOCATION as no longer required from Android 12. If your app depends on this permission, make sure you add it in your Android Manifest file.
• Fixes comments
• Removes @RequiredPermission annotation from startScan and stopScan methods, as the required permission now depends on the Android version. Native API fixed this issue by adding new annotations, but they are hidden and lint shows errors.