Add mm_event trace events to the atrace 'memory' category

Bug: 150130660
Test: Recorded a trace via the atrace commandline and verified
mm_event entries were included.

Merged-In: Ia5e13cf0aaaeb88c0c6efa68f89427b01af81f97
Change-Id: Ia5e13cf0aaaeb88c0c6efa68f89427b01af81f97
(cherry picked from commit f1fb849a)