Fix signature test issue caused by https://r.android.com/1613103

Change https://r.android.com/1613103 changed the name of the
instrumentation arg that the SignatureTest uses to retrieve the files
that define the expected API but did not change the corresponding
entries in all the AndroidTest.xml files that used that class, except
for CtsSystemApiSignatureTestCases's AndroidTest.xml file. That meant
that those tests that were affected ended up checking nothing.

This change switches back to using the previous name and reverts the
change to CtsSystemApiSignatureTestCases's AndroidTest.xml to use the
previous change.

This also adds some error checking to prevent this sort of bug from
arising again.

Bug: 196345428
Test: atest -p cts/tests/signature
      atest CtsHiddenApiBlocklistDebugClassTestCases
Merged-In: Ib9d1fcb62655534998aeb64e1c31f277e4ef9ed0
Change-Id: Ib9d1fcb62655534998aeb64e1c31f277e4ef9ed0
(cherry picked from commit b0db4cae905b93d9cf0169700483eda4f8314a70)
6 files changed