audio: 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
(cherry picked from commit 1cf494eddfef8f6c0c68dc28cd27203980d1767b)
(cherry picked from commit adf8475157fd8cba9ef527702f7b34f6335554eb)