Throw IllegalStateException when inserting an existing item

This commit throws IllegalStateException instead of
IllegalArgumentException when inserting an existing item
to TestBpfMap to fit the current behavior in the
production code.

This is safe given that this is only for test and no one is
currently check the exception.

Test: atest TetheringTests:com.android.networkstack.tethering.BpfCoordinatorTest
Bug: 354619988
Change-Id: I1e5c6e99a0d95b44dd556de8805bb16c3d1a4f61
1 file changed