Mark UserControllerTest.testStartUser_background as @FlakyTest

Bug: 118932054
Test: Pass all AM tests in WmTests
  $ tradefed.sh run commandAndExit WmTests \
      --include-filter com.android.server.am. \
      --include-annotation android.platform.test.annotations.Presubmit \
      --exclude-annotation androidx.test.filters.FlakyTest

Change-Id: I4fab1b08cc5061a2e107292e5d9bd6e2a6a121e5