NNXSW-1697 Remove use of SubgraphView that may become invalid.

The mainSubgraph variable held references to layers in the optimised
graph that become invalid when a graph substitution is made.

The fix is to always use the optimised graph itself rather than a
view of it. The view served no purpose anyway.

!android-nn-driver:1221

Change-Id: I579f503827db8f7fcb01443237c9d888d496425f
Signed-off-by: Rob Hughes <robert.hughes@arm.com>