Introduce a flag to enable shell caption

A new interface in WindowManagerProxy was introduced to let the shell be
able to set insets with a given frame. The interface won't work for
caption as we applied special logic to assemble the caption frame with
the attached task frame. To make it work with caption insets, we need to
disable all the special logics to make use of the shell interface.

A flag CAPTION_ON_SHELL is introduced to disable the calling path and
enable the above client owned logic. Turn the flag to true when the
caption is moved to shell.

Test: InsetsControllerTest, WindowContainerInsetsSourceProviderTest
Bug: 189998209
Change-Id: I989b319001a118d1f1ff8e34761b39ee18fde335