2159-Fix_session_validation_issue_for_dav_requests
Description
- Clear ownCloudClient instance & persistentCookie when user update his/her credential. It improves any unwanted behavior.
- Sometimes sessionCookie expired & doesn't renew on-exchange. In that case, we want to create new session & retry the sync
- Using app specific userAgent causes multiple session entry again same cookie in the web & backend against same sessionCookie. It causes confusion for user. Using nc-android-lib default userAgent to improve this behavior.