Use a new fixed-read-only flag to control the e2e feature

Accessing a fixed-read-only flag should be much faster. This CL is to
improve the performance.

The nav_bar_transparent_by_default flag is not needed because the
feature will be covered by the edge-to-edge feature. This CL stops
reading it.

Fix: 301622545
Bug: 309578419
Test: local override and tested with WindowInsetsTests
Change-Id: Id86e476353ef60d7db55256231d23d3608f8e0b6