Introduce FileRotator.

Utility that rotates files over time, similar to logrotate. There is
a single "active" file, which is periodically rotated into historical
files, and eventually deleted entirely. Files are stored under a
specific directory with a well-known prefix.

Bug: 5386531
Change-Id: I29f821a881247e50ce0f6f73b20bbd020db39e43