Add GetPathString

The method is a thin wrapper around GetWalkPath and GetTagPath to make
it easy to construct a string representation of the current path.

It was originally inlined in the apex package. This change makes it a
function and moves it to the android package so make it more useful.

Bug: N/A
Test: m
Change-Id: I7e2bc2074baed759d67d9097151c9ac10e34ed31