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
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