Cert: Use LOW_LATENCY 100ms adv in test_background_connection

LeAclManagerTest.test_background_connection fails with real
devices more often than it should.  When it passes, the
connection completes after 8 seconds.

Advertise more frequently to see if we can improve the
connection latency.

Bug: 199899668
Test: cert/run
Tag: #refactor
Change-Id: Ic9130f22e39bff1601537e43961be676b9a47567