Update the VDM documentation.

 - Add top level docs to VDM
 - Add docs to the aidl files
 - Document the differences between the two VirtualDevice classes
 - Extend VirtualSensorDirectChannelWriter docs
 - Fix minor inconsistencies, formatting and wording.

Fix: 269078018
Fix: 257449173
Test: n/a
Change-Id: Icb5f818f234f5eba03d7f685f58385aee8631000