Remove password shards from memory

Force a garbage collection and zeroize some fields after Activity finishes

Test: Goes through password change flow, then grab a heap dump via
      adb shell 'am dumpheap $(pidof com.android.settings)
      /data/local/tmp/settings.hprof'
      And grep for password in the dump
Bug: 144537463
Change-Id: Idd0a04ada98900aeb2a6d20bb1270a4a4aec2cfd