Inline "enable audio policies" flag

We cannot get rid of the flag, because it's exported. But we can assume
the value to be true, which has been the case since Android V. This
simplifies the system routing implementation considerably.

This change is expected to be completely non-functional.

Bug: b/391325457
Test: atest CtsMediaRouterTestCases
Test: atest MediaRouterServiceTests
Flag: EXEMPT code cleanup
Change-Id: I0c12d2b63b0c3969c91e0082f7821e342360b724