docs: update backup dev guide with Android Backup Service registration info
docs: update backup dev guide with Android Backup Service registration info

Change-Id: I16696abbd517a5a740ff641ca0b17ffafbcf4993

xorware/android_frameworks_base,xorware/android_frameworks_base,xorware/android_frameworks_base,xorware/android_frameworks_base,xorware/android_frameworks_base,xorware/android_frameworks_base