Fix flaky testSampleConnectivityState

testSampleConnectivityState was flaky since sometimes networks were
disconnected before ConnectivityService#sampleConnectivityState
generates metrics
This CL fixes this issue by adding setKeepConnectedReason to prevent
networks are disconnected

Bug: 318546530
Test: TH
Change-Id: I824f5cd379c8144b48341cc0fd18eb976a4b1cde