The MDS will be signed with platform key and become a platform app. To
make the selinux rules for modem_diagnostic_app work, need to set it to
platform app in app context.
Bug: 287683516
Test: Tested with both dev key or platform key signed MDS apps and the selinux rules works.
Change-Id: I375f57537a81514d3a6230ca042a4407accd6c15