Cleaning up flag NETWORK_BLOCKED_FOR_TOP_SLEEPING_AND_ABOVE

This flag should be enabled in the Android 15 release. Cleaning up the
flag so the value of true is used by code instead.

Note: No functional change is intended by the CL if the flag value is
considered to be true.

Flag: EXEMPT removing \
com.android.server.net.network_blocked_for_top_sleeping_and_above

Test: atest CtsHostsideNetworkPolicyTests
Test: atest FrameworksServicesTests:NetworkPolicyManagerServiceTest

Bug: 304347838
Change-Id: Ibe8b4226368b6ce17b8c1c084129924844391372