Unregister DeviceIdle Receiver in IpSecPacketLossDetector

Ensure that when the IpSecPacketLossDetector cleans up due to the
underlying network changing, that the broadcast receiver it's listening
to is cleaned up. Previously this would leak broadcast receivers.

Based on inspection, all instances that touch the BroadcastReceiver
are done on the same looper, so we shouldn't need to guard the creation
and destruction of the receiver.

Also, stop closing the monitored IpSecTransform. The monitor should not
have been closing the transform in the first place. Most of the time it
isn't an issue; however, in the event of MOBIKE, the transform will
remain open, so this is a functional bug.

Bug: 389208101
Test: atest IpSecPacketLossDetectorTest:testClose
Flag: EXEMPT bugfix
Change-Id: I26245960c36eaa0557d377b1672cc76ab0f70b03