Subscribe: support multiple subscribers to the same event

Different subscriber may subscribe to the same event.
This CL adds a hash map of a list of event subscribers
to support cross-device event.

Refators
- rename receiver to subscriber
- rearrange function ordering

Bug: 187548661
Test: CTS, GCA, Memery leak detection
Signed-off-by: Nick Chung <nickchung@google.com>
Change-Id: I65670796423c05b71f526062be60f45e3c7ea861