Fix NPE in flashlight button of energy widget

In somewhat unreproducible cases, pushing the flashlight button caused a
NullPointerException when trying to access the (not yet initialized)
mContext member variable in toggleState().

As I could not reproduce this reliably, and found no clear reason for the
variable to be uninitialized sometimes, but as there seems no need to
remember the context in a member variable anyway, I removed it and
refactored the methods to use a method parameter instead.

Change-Id: Id2b194f9f5785bcbbeb4bcd3d603510e0d0f71ce