[DK2-2]Add flag to control dynamic keepalive mode

Add a flag to control dynamic keepalive mode so that this
feature could be dynamically enabled via flag push. Default
is enabled in KeepaliveTracker.

Bug: 259000745
Test: atest FrameworksNetTests
Change-Id: I438de9aefd22229669a9ae4da5fd109fdfa73b10
2 files changed