composer: move LayerCommand to be part of DisplayCommand

This CL also includes a few miscellaneous changes:
 - Remove ..composer3.command namespace
   (so all commands are in composer3)
 - Minor documentation update
 - Minor name changes

Bug: 201008710
Test: VTS
Change-Id: I3fc24392d6c019c39e4d7df735abacc2c793c4f9