memevents: API to expose client's bpf ring buffer fd

Adding a new API to allow for a MemEventClient to fetch their
corresponding bpf ring-buffer file descriptor. This will allow clients,
such as LMKD, to insert the fd into their polling instance, without the
need to create a new thread to utilize the current `listen()` API.

Bug: 244232958
Test: atest memevents_test
Change-Id: Ie57499f25c8af8a636fd25b94c3f2217ea352158
Signed-off-by: Carlos Galo <[email protected]>
3 files changed