Use JobScheduler to run index update for API 21+
Currently F-Droid uses AlarmManager to run index update. This approach has some peculiarities, see 1(https://commonsware.com/blog/2013/03/08/warning-xperia-z-stamina-alarmmanager.html) 2(https://commonsware.com/blog/2013/03/11/more-xperia-alarmmanager.html) 3(https://commonsware.com/blog/2015/03/23/alarmmanager-regression-android-5p1.html) 4(https://commonsware.com/blog/2015/06/12/psa-another-alarmmanager-regression.html).
Android 5.0 introduced JobScheduler, that is promised to be more effective for such tasks. If it's employed, RECEIVE_BOOT_COMPLETED may be marked as android:maxSdkVersion="19".