Add config_canInternalDisplayHostDesktops to differentiate.

With the introduction of Extended vs. Projected Display, it is now
possible for devices to have Desktop Mode enabled but only on connected
displays, and not its internal display. Introducing a new config so we
can differentiate the two.

Bug: 384568161
Test: atest
Flag: EXEMPT bugfix

Change-Id: Id3e406b62219183ae33ac2cffb241db8457e1a6b