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
diff --git a/core/java/android/net/SocketKeepalive.java b/core/java/android/net/SocketKeepalive.java
index 46eddde..ec73866 100644
--- a/core/java/android/net/SocketKeepalive.java
+++ b/core/java/android/net/SocketKeepalive.java
@@ -44,9 +44,11 @@
  * {@link SocketKeepalive.Callback#onStopped} if the operation was successful or
  * {@link SocketKeepalive.Callback#onError} if an error occurred.
  *
- * The device SHOULD support keepalive offload. If it does not, it MUST reply with
+ * For cellular, the device MUST support at least 1 keepalive slot.
+ *
+ * For WiFi, the device SHOULD support keepalive offload. If it does not, it MUST reply with
  * {@link SocketKeepalive.Callback#onError} with {@code ERROR_UNSUPPORTED} to any keepalive offload
- * request. If it does, it MUST support at least 3 concurrent keepalive slots per transport.
+ * request. If it does, it MUST support at least 3 concurrent keepalive slots.
  */
 public abstract class SocketKeepalive implements AutoCloseable {
     static final String TAG = "SocketKeepalive";