add the LOCAL_DISABLE_TEST_CONFIG variable

Introduce and set the variable LOCAL_DISABLE_TEST_CONFIG varaible to be
true for the `android_test_helper_app` type modules.

Bug: 188927912
Test: rum 'm module-name' (`android_test_helper_app` type module)
Test: TreeHugger
Change-Id: I5db1029f31d8afe1ca5e9fab95e39d751971058d
Merged-In: I5db1029f31d8afe1ca5e9fab95e39d751971058d