battery : read fails /sys/class/power_supply/battery/capacity at the first access...

battery : read fails /sys/class/power_supply/battery/capacity at the first access time.

When the TW emulator first accesses /sys/class/power_supply/battery/capacity,
it sometimes returns an empty string.

So, lowbat_monitor() is called with 0 parameter.
"0" means that battery level is 0.
When deviced detects battery level 0, it creates a 30 second power off timer.
So the emulator automatically powers off intermittently after 35 seconds.

If an empty string is returned, we modify it to read /sys/clas/power_supply/battery/capacity again.

Signed-off-by: INSUN PYO <insun.pyo@samsung.com>
Change-Id: Ieefd950fe175ef138814624f6a3957ac7432cc74