Pin TARGET_PRODUCT to aosp_arm in integration tests

aosp/3019827 does not handle the case where integration_tests might be
run from an unlunched-terminal. In this scenario, the ninja file will be
generated at out/soong/build.ninja.

To support running integration tests with/without lunch as well for
consistency of tests across different setups (users/CI etc.), pin the
TARGET_PRODUCT to a default value (aosp_arm for now)

Test: build/soong/tests/run_integration_tests.sh without running lunch
Change-Id: I1197923867643986ed3736cfc284632aa82dca13