Force a garbage collection after lockscreen verification

This is to attempt erasing any lockscreen password remnants from memory,
which is a NIAP certification requirement.

Bug: 142861621
Test: reboot device, enter lockscreen password, then use pmdump to dump
process memory for SystemUi, verify password does not exist in the dump.

Change-Id: Ifbec81a64e4c592b566f5653bd93fd7326693ec7