Synchronize access to listeners in VDM.

Listeners are accessed from Binder thread when dispatching the
events, however registering / unregistering listeners is done
from different thread - this cl adds locks to prevent race condition
while unregistering / registering listeners while callback is being
dispatched.

Bug: 263231559
Test: atest CtsVirtualDevicesTestCases
Change-Id: I236dddcb25e406d5281a85da7d90bc13159b5a1f