Bug: 328704096
The soong config variable should be defined for all pixel products to
enforce backwards compatability (ensure that this is never built for non
enabled devices). In the case where `SHARED_MODEM_PLATFORM` is not
defined, then the soong config variable will choose the default
placeholder implmentation.
Doc: go/shared-modem-platform-cpp-backend
Test: Build for enabled and disabled targets and ensure that the
correct version of `shared_modem_platform` is being built.
Change-Id: I06ebdada2324fdd86cbb7718fd66602dd52070ea
c14a6a75cc