Support proxy method in StackVisitor::GetThisObject

Adds function artQuickGetProxyThisObject which returns the 'this'
object of the proxy method using the QuickArgumentVisitor. Since
proxy methods have the same layout than the kRefsAndArgs runtime
method and 'this' is the 1st method argument, it is located in the
first GPR.

Bug: 17965861
(cherry picked from commit a836bc97)

Change-Id: I09b5c1cdfc051d9395bba929d4650804eb3b1f7f