Fix VPN retry timer mechanism

This commit does:
- Skip the new retrying request when there is a pending retrying
  task. Otherwise, the mRetryCount will be increased unexpectedly
  before the current retrying task finishes its job.
- onChildOpened will call onSessionLost when MTU is invalid, and
  onChildOpened will be called after onIkeOpened, so if
  mRetryCount is reset in onIkeOpened, the mRetryCount will always
  start from 0.

Bug: 256776571
Test: atest FrameworksNetTests:VpnTest
      Manual test and check the log.
Change-Id: I396a3c279dc909f78c400cff1c31d77fe98cd7a6