Work around C11 const atomic restrictions.

Cast away the const qualifier in BBinder::findObject.  C11, unlike C++11,
does not allow loads from const atomics.  This is widely regarded as
a bug (see WG14 DR 459).  This is a hack to work around it until it's
officially fixed in the spec.

load_const_atomic was adapted from commit
1e8587a479fd8b1ce9b594298a93f517816e8f15

I don't think we want to dignify this by putting it into a header file.

Bug:17067219
Change-Id: Icbfcbda2722e6f80d2bb065a0bb3ec7634bcacb2