Add observer to bufferpool clients

Bufferpool clients are notified bufferpool events via FMQ messages.
Usually the messages are processed by pipeline via piggybacking. To
ensure process FMQ messages even if the process is in idle state,
Observer hidl interface is added. Buffer invalidation caused by
pipleline completion uses Observer interface in order to invalidate
buffers after all normal operations are finished.

Bug: 112203066
Change-Id: I2a4a172d78c6bcedcc809c559321929ebc91c30d