Add full-width caption on Shell.

This CL adds a default full-width caption implementation on Shell to act
as a stable caption implementation in freeform windowing environment,
since the desktop mode caption is not yet finalized.
Currently, this caption is used unless the desktop mode flag is turned
on.

Bug: 260276028
Test: atest WMShellUnitTests
Change-Id: I354330cfb959b453e382fed44d87ca7f0308a4e2