Update some descriptions and refine code

Address aosp/2052545 leftover comments:
- Log a wtf when the received user is unknown.
- Move the code to where it's used.
- PermissionMonitor is using BpfNetMaps for sending traffic
  permission. So it’s useless to do the netd null check before
  sending permission. Thus, remove this redundant check.

bug: 224775316
Test: atest FrameworksNetTests
Change-Id: I5c1291b6b855747d7900372b800dd039dd0730fe
1 file changed