Screenshots logging update

Unify logging tags and flags across screenshots
Log to single tag by default, with option for class name
Individual flags for categories of logs and a master on/off

Test: set LogConfig.DEBUG_ALL = true
      adb shell input keyevent SYSRQ
      adb logcat -s Screenshot:*

Change-Id: I728de8e8b77269d74d40f382fcd947e4e40520ae