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
ec4eb8f448