Cleanup non-flexible insets logic

This CL removes the old non-flexible insets logic, as well as the flags
to control the two solutions.

Besides, this CL removed the old logic in calculating nav bar position
which is in effect even after the flexible flag is on when the
navigation bar doesn't exists. Adjusted TestDisplayContent to make tests
working with the modification.

Bug: 161689946
Test: m checkbuild
Test: atest DisplayPolicyTests
Test: atest ActivityRecordTests
Change-Id: I04af809b5ff1ea43772a41d36ed6d97fb80ee02e