Use StartsWith for IpSec Iface Find

Address Code Review Comments on Change-Id:
Ibc66b014692a978ebfcc7856dcc26524618668a8

Use android::base::StartsWith() rather than
strcmp() for checking the prefix of a tunnel.

Bug: 74560705
Test: runtest -x netd_integration_test.cpp
Change-Id: I5c3b9067840cfea6273b3c83e2ff94e1f54d6c83