Add "power" to the list of default trace tags

This change modifies the list of default trace tags so that data from
power rails is collected. This change also updates the preference key
version so that the new default is set for all users who have opened
Traceur before.

Bug: 193827985
Test: Wiped and flashed to check that the power category is enabled by
      default, then took a trace to verify that ODPM data is collected.
Change-Id: I22583923bd070df728f057ba531198f4470e7ae5
Merged-In: I22583923bd070df728f057ba531198f4470e7ae5

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