audioflinger: fix effect disconnect deadlock

Fix possible deadlock when several EffectHandles on the same
EffectModule are destroyed simultaneously:
 A wp on an EffectHandle should not be promoted to a local sp
with ThreadBase mutex held as the EffectHandle destructor can be
called when the sp gets out of scope which will call
ThreadBase::disconnectEffect() and try to acquire the mutex.

Use raw pointers instead of weak pointers for the list of handles
on an EffectModule.

Bug 6679606.

Change-Id: Ice8b602fb03a7d363c44ce3dced8a53540d96270