Move the test of minimum supported keepalive slots to CTS

This change also enables log when keepalive is started.

Bug: 134352656
Test: 1. atest android.net.cts.ConnectivityManagerTest#testSocketKeepaliveLimitTelephony
      2. atest FrameworksNetTests
Merged-In: I408750fa0bceb0c1c26afb5fead4e44fb824fbc1
Change-Id: Ib9b06d3458782a648a2f5b8fd5a9ca1d34bd11f9
(cherry picked from commit aed835f086440f721c279e33d6b5d4b4bc270296)

• core/java/android/net/util/KeepaliveUtils.java[diff]
• services/core/java/com/android/server/connectivity/KeepaliveTracker.java[diff]
• tests/net/java/android/net/util/KeepaliveUtilsTest.kt[diff]