Minor fixes for netd restarts and StrictController.

Minor fixes for netd restarts and StrictController.

1. Ensure we don't change strict mode network policy for a given
   UID from a non-accept policy to another non-accept policy,
   as netd does not support this.
2. Move the "strict enable" and "bandwidth enable" commands
   inside the lock. This improves correctness, and it is safe to
   do now that those commands now only take a few milliseconds,
   instead of several hundred milliseconds.
3. Fix an NPE in connectNativeNetdService which causes the system
   to crash when netd crashes.

Bug: 28362720
Test: bullhead builds, boots
Test: "adb shell killall netd" no longer crashes the system
Change-Id: Icdaa9d1e2288accf35de21df56bc6bd2b0628255