Temporarily disable `testBackgroundDexOptDowngradeSuccessful`.

Test
`com.android.server.pm.BackgroundDexOptServiceIntegrationTests#testBackgroundDexOptDowngradeSuccessful`
has been failing for a long time; temporarily disable it while we
investigate this issue

Test: atest BackgroundDexOptServiceIntegrationTests
Bug: 251438180
Change-Id: I3584ed2fd49e673a2d9bb37aabb11577c52a6267