Address leftover comments

- Add comment to highestPermissionForUid()
- Add testAppsAllowedOnRestrictedNetworksChangedMultipleUsers
- Add synchronized to guard mUidsAllowedOnRestrictedNetworks
  access.

Bug: 189705071
Test: atest FrameworksNetTests
Ignore-AOSP-First: Need cherry-pick
Change-Id: I9056758db15e8a21b84ef244d4cacf24c3b79894
2 files changed