This reverts commit ea3e3def

* refactored dav4android to use XML streaming
* refactored collection detection
* refactored sync logic

* asking for permissions is already (and better) done by AccountActivity

* don't show DAVdroid icon unless there's a strong relationship to DAVdroid itself

* refactor checking for cancelled sync
* notify on SSLHandshakeException (except when a certificate was rejected by cert4android)
* show exception cause in debug info

* raise API level to 19 (required by ical4j)
* make HttpClient use Settings to get sync working again
* don't use vector graphics for notification icons (crashes on Android 4.x)
* various fixes and improvements
* library and build tools updates

* ask for confirmation when group method is changed in account settings activity
* reset address books/force reload when contact group method has changed since last sync