Fix flakiness in LeAdvancedScanningTest lost test

Wait for scan result match found event to ensure scanner is started before stopping advertiser to generate lost match event. Otherwise the scanner may not have started before advertising stops on the cert side and the test may fail to catch it.

Bug: 232424520
Test: gd/cert/run --device --clean --sl4a
LeAdvancedScanningTest.test_scan_filter_lost_random_address_with_irk
--repeat=10
Ignore-AOSP-First: Fixing flakiness on tests needed for presubmit on
tm-dev.

Change-Id: Id7957d989ffc95c7c869132d1f059957f5720fe2