Introduce InsetsSource#FLAG_FORCE_CONSUMING

It controls whether the insets provided by the source should be forcibly
consumed.

We have mForceConsumeSystemBars to achieve the similar logic, however,
it cannot control individual types. And when it is changed, there is no
guarantee that the new value will be sent to the clients. This CL fixes
the issues.

After this CL, the legacy "alwaysConsumeSystemBars" will be redundant,
and there will be a follow-up CL to clean it up.

Fix: 277891341
Test: Launch immersive apps in kids space and make sure their content
      wouldn't be obscured by navigation bar.
Change-Id: I1c8b628660d7cde167f5bc832f8cc7568ab87d61