SystemUI: refactor fast charge toggle

Device trees need to set a path to enable the toggle.
It will not show otherwise.

Moved all file operations off the main thread and use a
preference to save the last state so it persists.

	<!-- empty string to disable, or like: /sys/kernel/fast_charge/force_fast_charge
         it is disabled by default so devices need to add an overlay to enable-->
    <string name="config_fastChargePath" translatable="false"></string>

PS8: Lower delay to 250

Change-Id: Ib1465a98893ec8e7854c8b80f8352016ff6248ad
Signed-off-by: Roman Birg <romanbirg@gmail.com>

Conflicts:
	packages/SystemUI/src/com/android/systemui/statusbar/policy/Prefs.java