Add IVirtualInputDevice.

Step 1 of decoupling virtual input from VDM: Factor out the input
related logic from IVirtualDevice to a new interface.

Purely no-op, just moving stuff around.

Bug: 419493538
Test: atest & presubmit
Flag: EXEMPT no-op refactor
Change-Id: I88a06a0346ee699327f303a45f52b06b8635189b