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