charger: Draw battery percentage over charger screen

Let user know the exact battery capacity.

Port from https://github.com/CyanogenMod/android_system_core/commit/f7e1ef2461121c1ebe7f1ef3ea25cca397e4c5ab

Change-Id: I841efe8fa516cbd565e06fc2bd640ce7e3030d0b
Merge remote-tracking branch 'omni/android-4.4' into android-4.4