Add missing 'unset' in config flag vectorizing

[almuthanna-athamina]

Fix multi-instance default flag resolution in config_flag.cpp by populating missing instance flags with "unset". This ensures that unspecified instance flags fall back to their per-instance defaults rather than inadvertently replicating the primary instance's settings.

Bug: 354927775, 543955575

[jmacnak]

Note: ag/41631463

[Databean]

There's a failure in GitSwCarDevSdv, which I was able to reproduce locally:

$ cat $HOME/sdv_config.json
{
  "instances": [
    {
      "name": "ins-1",
      "vm": {
        "cpus": 2,
        "memory_mb": 2048
      },
      "boot": {
        "extra_bootconfig_args": "androidboot.sdv.instance_name=instance1 androidboot.virt.address=3 androidboot.sdv.boot_mode=unlocked"
      },
      "security": {
        "guest_enforce_security": false
      },
      "disk": {
        "default_build": "@ab\/git_main-swcar-dev\/aosp_cf_x86_64_sdv_core-trunk_staging-userdebug"
      },
      "graphics": {
        "gpu_mode": "none"
      }
    },
    {
      "name": "ins-2",
      "vm": {
        "cpus": 4,
        "memory_mb": 4096
      },
      "boot": {
        "extra_bootconfig_args": "androidboot.sdv.instance_name=instance2 androidboot.virt.address=4 androidboot.sdv.boot_mode=unlocked"
      },
      "security": {
        "guest_enforce_security": false
      },
      "disk": {
        "default_build": "@ab\/git_main-swcar-dev\/aosp_cf_x86_64_sdv_media-trunk_staging-userdebug"
      },
      "graphics": {
        "displays": [
          {
            "width": 1920,
            "height": 1080
          }
        ],
        "gpu_mode": "gfxstream_guest_angle_host_swiftshader"
      }
    }
  ],
  "netsim_bt": false,
  "metrics": {
    "enable": true
  },
  "common": {
    "host_package": "@ab\/git_main-swcar-dev\/aosp_cf_x86_64_sdv_media-trunk_staging-userdebug"
  }
}
$ bazel run //cuttlefish/package:cvd -- load $HOME/sdv_config.json
/var/tmp/cvd/270178/1787611512682778/artifacts/host_tools/bin/assemble_cvd --helpxml returned unexpected response -1. Stderr was assemble_cvd failed:
7. assemble_cvd.cc:663 | AssembleCvdMain | Failed to parse flags.
6. feature.cpp:69 | ProcessFlags | Unable to parse flags.
5. feature.h:169 | TopologicalVisit |
4. feature.h:165 | operator() | Callback error on DisplaysConfigsFlagImpl
3. feature.cpp:65 | operator() |
2. display.cpp:84 | Process |
1. display.cpp:136 | ParseDisplayConfigsFromArgs |
 | cuttlefish/host/libs/config/display.cpp:56
 v Result<std::optional<CuttlefishConfig::DisplayConfig>> cuttlefish::ParseDisplayConfig(const std::string &)
Expected "keyvalue.size()" == "2" but was 1 vs 2. Invalid display flag key-value: "unset,width=1920,height=1080,dpi=140"