Build mode function should return output file

Each build mode has to provide an output file, which also has a
corresponding dependency file (<output file>.d). Previously, some of
the build mode functions returned that file, others just returned and
relied on the doChosenActivity() method to return the file.

That was confusing as the same file name was referenced from two
separate places. This change makes all the build mode functions return
their output file explicitly and the doChosenActivity()just returns
what it receives. It also moves the writeDepsFile call to adjacent to
the return.

Bug: 257650737
Test: m nothing
Change-Id: I35f96ab404d9ffb1da747a3ecb8fc4e1feb9c026