Security Logging: log password change events

* Log when the user's lockscreen password has been changed, together
with the new password's complexity level.

* Also expose SecurityLog.writeEvent() as SystemAPI, as some
mainline modules (WiFi etc) will start  emitting security events.

Bug: 194988881
Test: atest MixedDeviceOwnerTest#testSecurityLoggingWithSingleUser
Test: atest OrgOwnedProfileOwnerTest#testSecurityLogging

Change-Id: I2da4fdd15f85fab9244a978be7b1dafb4a82d190