Check mm_events persistent property after persistent properties are ready during boot.

Test: adb shell setprop persist.mm_events.enabled true; adb reboot \
      adb shell ls /sys/kernel/tracing/instances/mm_events -> Exists
Fixes: 324600722

Change-Id: I091ca35f48e4baf8b6b5c24fba325f1dcae6b8b7