Support customization of supported keepalive count per transport

This change specifies the required minimum supported keepalives
in SDK, and allows OEMs to customize supported keepalive count
per network through resource overlay.

Bug: 129371366
Test: 1. m -j doc-comment-check-docs
      2. atest FrameworksNetTests

Change-Id: I0218f3674628c13ead63fc9a873895ba7f113033
diff --git a/core/java/android/net/SocketKeepalive.java b/core/java/android/net/SocketKeepalive.java
index 9d91620..46eddde 100644
--- a/core/java/android/net/SocketKeepalive.java
+++ b/core/java/android/net/SocketKeepalive.java
@@ -43,6 +43,10 @@
  * To stop an existing keepalive, call {@link SocketKeepalive#stop}. The system will call
  * {@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
+ * {@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.
  */
 public abstract class SocketKeepalive implements AutoCloseable {
     static final String TAG = "SocketKeepalive";