This commit includes the major changes between DAVdroid and EteSync. It
adjusts the app to use the EteSync protocol and server. It includes some
ugliness still, and it's a squash of many ugly snapshot commits while
hacking on the initial DAVdroid code.
History should be "clean" from this point onwards.
* use cert4android instead of MemorizingTrustManager
* new app setting: distrust system certificates
* add network security config to manifest so that user-installed CAs will be accepted in Android 7 again
* update gradle
* use weak references for DavService RefreshingStatusListener
* additional null checks for cases which shouldn't appear, but apparently appear
* additional database conflict handling for cases which shouldn't appear, but apparently appear
* setup by URL: null check for empty host names
* vcard4android: upgrade to ezvcard 0.9.10
* AccountSettings$AppUpdatedReceiver: check for migrations only when package is replaced
* SyncAdapter: move DB helper from service to SyncAdapter to prevent databases from being closed too early
* Manual sync button: run sync immediately (without queueing)
* handle and show OutOfMemoryErrors correctly (they're not Exceptions)
* use db.beginTransactionNonExclusive() because WAL is enabled
* set calendar VISIBLE=1 AND SYNC=1 only at creation and not at every sync
* update PendingIntent of service refresh notification
* add InvalidAccountException for invalid (=not existent/invalid settings version) accounts
* handle invalid accounts properly
* HttpClient: add constructors without Account when authentication is not needed
* drop upgrade compatibility for accounts without version (version<1)
* add startup dialogs (F-Droid: donations, Play Store: DRM bug, OpenTasks not installed)
* allow to reset hints/startup dialogs
* AccountSettings: fragment as inner class of activity
* CalDAV/CardDAV resource list views now always contain all elements without scrolling (NonScrollingListView)
* synchronization action in activity now overriddes system sync settings