frameworks/base: Fix to remove exception in tonegenerator

- Framework reboot happens, if tonegenerator fails to create
  a track in audioflinger.
- The cause for framework reboot is, an exception thrown by
  tonegenerator if fails to init.
- Fix is to remove the exception and delete the tonegenerator
  instance.
CRs-fixed:466491

Change-Id: Ie7b87f787d8a885cc70be9af4ba9d04ecd691cc2