memshare: Move ramdump collection to AFTER_SHUTDOWN event

Collect the ramdumps of memshare clients when notified of
SUBSYS_AFTER_SHUTDOWN event instead of SUBSYS_RAMDUMP_NOTIFICATION
to align with the deprecation of the SUBSYS_RAMDUMP_NOTIFICATION
event.

Change-Id: I303ab90a39843f837547a2f817cefa52c75063f3
Signed-off-by: Manoj Prabhu B <bmanoj@codeaurora.org>