Do not transform again on crash

Before:

onServiceConnected
   AsyncTask 1: doInBackground
                doTransform
                   PdfEditor service crashes

system restarts PdfEditor service -> onServiceConnected
   AsyncTask 2: doInBackground
                doTransform
		onPostExecute
		   Unbind from service

   AsyncTask 1: onPostExecute
                   Unbind from service again -> crash

Hence we have to only ever create a single async task even if we get
another onServiceConnected.

Test: atest CtsPrintTestCases
Change-Id: Ia18e82fe6258bac9395557b2056645e87a752889
Fixes: 73127227