Add cleanup method to FalsingManager to prevent memory leaks.

When the FalsingManager gets reloaded (due to plugins) it can
leak its listeners and callbacks. This change fixes that.

This is a CP of http://ag/8668802.

Bug: 136351609
Test: manual
Change-Id: I7ea8384678b60e78ed384e19bbd7932722fe2b9c
Merged-In: I2b52d018d478dbcad4ecb7d8a5b361638d5c5877