Construct a default AdvertisingConfig in tests

Since AdvertisingConfig is a non-aggregate type, it causes an error to initialize
it with a designated initializer list.

Bug: 305066880
Test: mma -j32
Test: bluetooth_test_gd_unit \
            --gtest_filter=LeExtendedAdvertisingManagerTest
Flag: EXEMPT, test fix for an infrastructure change
Change-Id: I069b45f8abb2101f1a383dee5761709e631aa792