DO not throw exception for ERROR_NO_SUCH_SLOT error

There are multiple independent reasons a keepalive can stop. Some
are software (e.g. the app stops the keepalive) and some are hardware
(e.g. the SIM card gets removed). Therefore, there is a very low
probability that both of these happen at the same time, which would
result in the first stop attempt returning SUCCESS and the second
stop attempt returning NO_SUCH_SLOT. Such a race condition can be
ignored with a log, not to crash the system. Change to wtf log so
that the issue could still be visible since this implies possible
bugs in the system.

Also update the javadoc for SocketKeepalive.ERROR_NO_SUCH_SLOT.

Bug: 281484381
Test: atest FrameworksNetTests
Change-Id: I316a2193fc7302b04e074ff0b03dc43946fe5ce6