Move imageMutator before archMutator

Run the imageMutator between osMutator and archMutator so that
different arch variants can be set for the different partitions.

Bug: 142286466
Test: m checkbuild
Change-Id: I65d05714b75aa462bf9816da60fdc2deda4de593
Merged-In: I65d05714b75aa462bf9816da60fdc2deda4de593
(cherry picked from commit 9c8f6875)