Release the mutex before calling hidl_cb

In Omx::allocateNode, Omx::serviceDied may be called by hidl_cb in
Omx::allocateNode if the observer dies before hidl_cb is called. This CL
prevents a deadlock from happening in this situation.

Test: make cts -j99 && cts-tradefed run cts-dev \
--module CtsMediaTestCases --compatibility:module-arg \
CtsMediaTestCases:include-annotation:\
android.platform.test.annotations.RequiresDevice
Bug: 63177989

Change-Id: I8e1357ff637b0912a3a97c6611d886533caaaf8a