BatteryStats: Read /proc/wakelocks first before trying /d/wakeup_sources

Previously we tried to read /d/wakeup_sources to gather kernel wakelock data. If that
fails we used the older sys file /proc/wakelocks.
N7 has both /proc/wakelocks and /d/wakeup_sources, but /proc/wakelocks
has the actual data we need. All other devices are using /d/wakeup_sources, so
only N7 experienced a loss of kernel wakelock data.

The original regression was introduced here: ag/659258

Bug:22556242
Change-Id: I51ec68e957f587bc1466e24f0a1dbc8cd7753ac6