Simplify DisplayManager flag library

Now all the flags are read-only, we don't need to intercept the
exception, as it cannot happen anymore.

Note, there is one flag that is read-write, but it is not used anymore
and will be cleaned up soon.

Fix: 297159910
Test: Manual
Change-Id: Iac1e22bd292f02bee7a5cc4fce6ae618dfe0258f