LWIS: Optimize IRQ process time

Generic sensor will subscribe CSI kFrameStartEvent(2000000100000102)
which is extra processing time(20 ~ 30us) here.

This change is to add a tasklet in top-device and then this tasklet
will forward the external events. Can reduce 30~-40us IRQ process time.

Bug: 182290526
Test: CTS, GCA portrait mode with no event delay
Signed-off-by: Nick Chung <nickchung@google.com>
Change-Id: Ia05737b6c84a47f9e0a2ad4c6377c9edf3ef6506