Add a system property to init that drops caches.

*   Users can't call /proc/sys/vm/drop_caches otherwise.
*   Requires change with associated SELinux permissions.

Bug: 178647679
Test: flashed user build and dropped caches; no avc denials.
Test: flashed userdebug build and dropped caches w/o root.
Change-Id: Ibc91991509f0902d1c04c958fefc0966233f0b2e