fix deadlock in audioflinger::createEffect()

commit 5baf2af5 introduced a regression by calling
getOutputForEffect() with AudioFLinger main mutex locked.
The locking order must always be AudioPolicyService mutex then
AudioFlinger then ThreadBase mutex.

Bug: 10916796.
Change-Id: Ide34a2d84dbb06dbb35abd0640d91b01b0ac4d40