Request cell in #testExplicitlySelected

When the test is very slow (e.g. in coverage tests) it's
possible this test takes > 30 seconds and cell finishes
lingering before the end of the test. This will cause a
disconnection of cell before wifi and fail the test.

Address this by requesting cell.

Bug: 273383734
Test: atest --iterations 100 'FrameworksNetTests:android.net.connectivity.com.android.server.ConnectivityServiceTest#testExplicitlySelected'
Change-Id: I06a7ef82de966b522e58a969c25a4c0335dbc497