Add setSubscriptionOverride() to system API

Add new API setSubscriptionOverride() in NetworkPolicyManager
and rename constants OVERRIDE_* to SUBSCRIPTION_OVERRIDE_*.
Make them @SystemApi for mainline support.

Bug: 138306002
Test: atest FrameworksNetTests
      atest FrameworksTelephonyTests

Change-Id: I56c777aa66d6f455695f133f9889979c13cd1bc8