Freezer kill reason in App Process Kill Stats

The reason code is helpful for developers to understand why an app is
killed. It is also valuable to better quantify the benefit and impact
of App Freezer.

Bug: 183714099
Bug: 179715791
Test: dumpsys activity exit-info shows subreasons.

Change-Id: Iea808cdc8adbe9d84663071dc849d00ea28a9f7d