Refactor shared logic to VirtualInputDevice

VirtualKeyboard, VirtualMouse, VirtualTouchscreen and VirtualDpad share
some part of their logic. With the future addition of VirtualDpad even
more code is duplicated. This extracts the shared methods and fields
into an abstract superclass called VirtualInputDevice.

Bug: 244516415
Test: atest FrameworksCoreTests:android.hardware.input and atest FrameworksServicesTests:VirtualDeviceManagerServiceTest
Change-Id: I97acabe9b2f655dddbe179401a7833949efcd64b