bufferpool2: add sync() method and etc

Add sync() method to IConnection interface.
Add existing connection flag to return parcelable of
IClientManager::registerSender().
Use plain integer for returning fetch result status.

Bug: 254050250
Merged-In: Ifebd4b16e9b8c37e074e7126f39264a18fed5de8
Change-Id: Ifebd4b16e9b8c37e074e7126f39264a18fed5de8
6 files changed