AndroidTestUtilities: do not require ExternalData unconditionally

Attempting to use AndroidTestUtilities to simply install some local
files on device can result in the following error:

    Neither ExternalData_URL_TEMPLATES nor ExternalData_OBJECT_STORES is set

We no longer require these ExternalData-specific variables to be set if
no such remote data files are requested.

Fixes: #16529
Status Job ID Name Coverage
  External
passed ghostflow-branch-check

passed ghostflow-commit-check