Slight cleanup for UdfpsController

Create an internal ServerRequest class to keep track of per-request
things. Otherwise we will just have more and more global variables,
such as "mIsOverlayRequested", "mRequestReason", "mEnrollHelper",
etc, which is error prone and hard to understand/maintain.

Bug: 178403701
Test: atest com.android.systemui.biometrics
Change-Id: I2f446263f530741d76f40fa2d1470a0c209b7d5e