Add "network" as a default trace category

This change enables the network category by default, because additional
network/modem tracing has recently been added. Tests were carried out
with this change cherry-picked to an internal tree.

Test: - Build and flash (both user and userdebug)
      - Check that "network" is already enabled
      - Check that traces contain RIL tracks
Bug: 245975537
Ignore-AOSP-First: Cherry pick to tm-qpr-dev.
Change-Id: I36ea1d760503cdeb44c91caa15dc3aef838c6cb9
Merged-In: I36ea1d760503cdeb44c91caa15dc3aef838c6cb9

• res/values/preference_keys.xml[diff]
• src/com/android/traceur/Receiver.java[diff]