keymint: Specify ICommService accessor name for security VM

This cl renames the accessor instance for ICommService from
"default" to "security_vm_keymint" to make its purpose
explicit and unique.

The new name clearly indicates that the accessor points to the
ICommService instance exposed by the security VM, specifically
for KeyMint.

Bug: 404289721
Test: launch KeyMint connecting to trusty VM
Test: atest VtsAidlSharedSecretTargetTest
Flag: EXEMPT this change only affects Trusty qemu
Change-Id: Iaba785f95c935f35003d339babe3cc7f7cf6d80d