Refactored CustomVirtualView to be more generic...

...and moved the username/password logic to the VirtualSignInActivity.

Also:

* Changed the Clear button to cancel the autofill context.
* Put compile-time guards on debug and verbose logging statements.

Bug: 65384186
Test: manual verification

Change-Id: Ibd0138193e098a82b47ba5dd40658670853b91f8