Correct value of supported keepalive count for cellular

Cellular only supports 1 keepalive connection so correct the default
supported value to 1.

Bug: 134037217
Test: - atest FrameworksNetTests
      - atest android.net.cts.ConnectivityManagerTest \
        #testSocketKeepaliveLimitTelephony

Change-Id: Ia3761f2e78d54866bb2e156d58004396bbc8adc3