Using defensive copy for default request remove

Passing in a defensive copy for the default requests in
ConnectivityService prior to updating that collection so as to not have
issues when removing elements as part of the per-app preference update
flow.

Bug: 176494815
Bug: 178632672
Test: atest FrameworksNetTests
atest FrameworksNetTests:com.android.server
.ConnectivityServiceTest#testSetOemNetworkPreference
ClearPreviousOemValues --iterations

Change-Id: Ia683d5035dda2be35fd2f2b7ee038e806caab6f9