Initial buffering of Content Capture events.

IntelligenceManager must buffer ContentCapture events and send them to the
service in a batch, and this is the initial implementation of such batch:
it's just batching a pre-defined number of events, without any further
optimization (like flushing after x ms).

Test: manual verification

Bug: 111276913
Bug: 119220549

Change-Id: I96a4708fd3fcfd3098a0894a3ae3e967804cf4e6