Aapt2 ValueTransformer

For future macro support, aapt2 must be able to convert Reference
values into other Value types. Currently a DescendingValueVisitor is
used to visit all of the References in a ResourceTable or a compiled
XML file to set their resource ids during the link phase. This was fine
since we were only mutating the resource id of the visited Reference.

A macro may reference a String, BinaryPrimitive, or any other Item
type. During the link phase, we will need to transform references to
macros into the values of the macros.

The only parameter in the methods of the ValueVisitor interface is a
raw pointer to the type being visited. The visitor interface does not
support reassigning the visited type to a different type.

ValueTransformer is a new interface for consuming a Value type and
transforming it into a compatible Value type. This change refactors
Value::Clone to use this interface.

Bug: 175616308
Test: aapt2_tests
Change-Id: Ic1b9d718b932c208764114cd9c74d880e189ccb0