Use GraphViewer.IsConstantInitializer in NNAPI EP. (#19401)
### Description
<!-- Describe your changes. -->
An overridable initializer should not have a fixed value included in an
NNAPI model as it could be changed at runtime. The current check doesn't
include validating that the initializer is constant.
I was updating GetClipMinMax as part of adding CoreML EP ML Program
support, and in order to make both CoreML and NNAPI do the more correct
thing of using IsConstantInitializer this set of changes was required.
### Motivation and Context
<!-- - Why is this change required? What problem does it solve?
- If it fixes an open issue, please link to the issue here. -->
Make NNAPI and CoreML EPs more correct.
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Co-authored-by: Edward Chen <18449977+edgchen1@users.noreply.github.com>
