Removed ping request from initialize() when access token is already known

The subsequent updateInstallationCoreProperties() request will check the
access token validity and sync time anyway.
JSONObject.optString() returns "" by default + more null checks
Handle link button action
Handle rating button action
Handle trackEvent button action
Use stantard MIME type "vendor" format
Handle `method` button action
Don't show the notification again when bringing app back to front
isReady() tells if INTENT_INITIALIZED has already been sent out