Add checks to ensure SPIs are not reused

This change adds an additional check in CheckIpsecConfig to prevent
users from using the same SPI twice. This allows for a more granular
error message.

Bug: 70642141
Test: Tests added in IpSecServiceParameterizedTest
Change-Id: I9621fb05c6b162bd8ae8db4ac1e64feaa9d0ac73
2 files changed