Log metric when password complexity is set

Log a new metric: Calls to
DevicePolicyManager.setRequiredPasswordComplexity.

This metric is used to track API usage for the new method (to gauge
adoption).

Bug: 165573442
Test: atest com.android.cts.devicepolicy.MixedDeviceOwnerTest#testPasswordMethodsLogged
Change-Id: Ib2c3d9925dc33b5ea88d2c7d3b160520e937d1ad