Fix keepalive don't get removed when lower layer error

Currently, if the lower layer, e.g. wifi, didn't successfully
start keepalive by any reason. Due to the startedState changed
to NOT_STARTED first, the logic inside stop() will skip the
removing process and cause leak.

Thus, moving the changing of startedState to proper place first
to unblock subsequent changes first.

Bug: 123988249
Bug: 129371366
Test: atest FrameworksNetTests
Change-Id: I4bba01bacc80e1dac2023ef831b5ade5501894e4