Deflake doTestPlatformVpnWithException related tests

This commit updates test to
- Execute IkeSessionCallback inside executor. The callback should
  be called from executor in production code but test trigger the
  callback from test main thread which will result in unexpected
  code interaction.
- Verify the timer directly. The origin design verifies
  interactions with Captor with uncertain interaction. Test
  should be able to verify the schedule() calls with the expected
  timeout value. Also, in lower performance device, device may
  still process previous interaction. Add longer timeout to reduce
  flakes.

Bug: 293233390
Test: atest FrameworksNetTests
Test: atest VpnTest#testStartPlatformVpnFailedWithRecoverableError
      --rerun-until-failure 100
Change-Id: Ie42a2597a382800d6484a086b6d808490dfbdd32