Add a factory for PreviewPositionHelper

Enables testing as well as serves as a way to convey to clients that it
is stateful and hence a factory may need to be used to get a new object
for distinct invocations.

Bug: 399641570
Flag: EXEMPT BUGFIX
Test: N/A
Change-Id: I8411c4f8ef110128f9d59a24a5b5705bbf9abf7b