OMXNodeInstance: use a lock in freeNode

This is to avoid a concurrent use after free if other OMX commands
are being executed before the node is marked as deleted.

Bug: 63666573
Change-Id: I7720dd900bfa252f8675e0c56191adbf52aa957e
(cherry picked from commit 975fa5b8b6fcaffc7ade6ee4de56847b1839797c)