Introduce insets decoupled configuration feature flag

To be used to deliver a stable configuration decoupled from the insets.

With the decoupled configuration, the app can have more stable running
environment, and will not be affected by the increasingly complex insets
environment of the system.

Bug: 151861875
Test: m services.core.flags-java
Test: m services.core.unboosted
Test: m dist
Change-Id: Ib0c2ea93757eba99d66ed5c0cb798b84f4a9eccb