Rename sysprop for MTE mode in system_server.

Move the system property that controls MTE mode in system_server out of
arm64.memtag.process namespace. Process sysprops affect the MTE mode of
any process with the given name; using such property in a product means
that any process or test unfortunately named "system_server" would be
affected.

Bug: 280343521
Test: debuggerd `pidof system_server` | head -30 | grep tagged_addr
      affected by the new property
      not affected by the old property
Change-Id: I1b303803cd470808d493f8fa20a69e84b3906afc