Minor fixes to NetworkCapabilities#toString.

1. The current code only prints the array of administrator UIDs
   if it's empty. This is clearly an oversight. Print it only if
   it's non-empty.
2. Only print requestor UID and package name if they are set.
   This makes output shorter in the common case that they are
   unset.
3. Reorder the output at the end: group all UIDs together, and
   place SSID and private DNS broken bit after that.
4. Make the private DNS broken indication a single word instead
   of a sentence. This saves space and makes it easier to write
   regexps.

New format:
... SignalStrength: -72 OwnerUid: 1000 AdminUids: [1000] SSID: ...
... Uid: 1000 RequestorUid: 1000 RequestorPkg: android ...

Test: manual
Change-Id: I2f5ccc1d9e4af6ddacc4d193185a17723822972b