NotificationPanelViewController: separate DEBUG flag for drawable

ag/17958675 added some debug logging to NotificationPanelViewController,
gated by the class-level DEBUG flag, and changes that flag to be set to
Log.isLoggable(TAG, Log.DEBUG).

Unfortunately, that class also contains *visual* debugging, which was
enabled by the same DEBUG flag, and so any device with sufficiently high
log levels unexpectedly started displaying visual debugging info.

This change splits the flag into two: DEBUG_LOGCAT for textual logs, set
to Log.isLoggable(TAG, Log.DEBUG); and DEBUG_DRAWABLE for visual
debugging, hard-coded to false.

Bug: 233720721
Test: manual; visual debugging only appears with DEBUG_DRAWABLE true.
Change-Id: I78b120aa0750605c96f1f53d3decd06919496f0c