atrace: Setup throttled rss_stat event

Create a synthetic rss_stat_throttled event that is triggered from the
rss_stat event every time the rss size crosses a 512KB boundary.

Bug: 145972256
Test: ls /sys/kernel/tracing/events/synthetic
      cat /sys/kernel/tracing/events/kmem/rss_stat/trigger
Change-Id: I653397f756c60fa274f1e0198974d868945b8d99