Event: Put a cap on the number of events in pending queue

This is to avoid allocating an excessive amount of memory in case of an
interrupt storm.

Bug: 189262964
Test: CTS and GCA
Signed-off-by: Edmond Chung <edmondchung@google.com>
Change-Id: I681264465ce6e78e765db7e68d46df7a324311af