[1/2] Improve flash availability check.

* Use simple overlay, default false.

PS2: Add header and proper symbol.

Signed-off-by: Carlo Savignano <stevewatersy@gmail.com>
Change-Id: I40458959d738fbd82941a19ff1ffcfa236bffcb9

Conflicts:
	core/java/android/provider/Settings.java
	core/res/res/values/velocity_symbols.xml
	packages/SystemUI/src/com/android/systemui/statusbar/phone/QuickSettingsModel.java

Conflicts:
	core/res/res/values/velocity_symbols.xml

Conflicts:
	core/res/res/values/boost_symbols.xml