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

Clean cherry-pick of aosp/946359

Change-Id: I06840834d0ee8121358bf4829fe47ecf9964d395
Merged-In: I0218f3674628c13ead63fc9a873895ba7f113033
Merged-In: Ia667386c1a8949839871a6949d79552d9c8b88f0