Updates for Svc/DpWriter corresponding to UT cmake udpate for protected/private

[m-aleem]

Related Issue(s)	#3446
Has Unit Tests (y/n)	Yes - Existing
Documentation Included (y/n)	N/A

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Change Description

I locally updated fprime/cmake/settings.cmake to change -DPROTECTED=protected -DPRIVATE=private. This causes additional build errors beyond all our updates in manual code for PRIVATE->private and PROTECTED->protected due to auto-coded files which use PRIVATE, PROTECTED. This set of changes is associated with fixing Svc/DpWriter so that the UTs in this directory can build and pass.

Approach in this case was:

1. Add accessor methods to DpWriterTester

Rationale

#3446

Future Work

1. Note that this PR does not include the update to fprime/cmake/settings.cmake itself - this will be done at the end
2. Note that this PR does not include re-naming the flight code Impl to (separate issue)