add script to help override flag value locally

Add a script to help to create flag value building rules.

The script will only extract necessary information from the value file
but it will not validate the file. The validation will defer to building
system.

For input

```
flag_value {
    package: "com.android.aconfig.test"
    name: "disabled_ro"
    state: DISABLED
    permission: READ_ONLY
}
flag_value {
    package: "com.android.demoapp.test"
    name: "enabled_ro"
    state: DISABLED
    permission: READ_WRITE
}
flag_value {
    package: "com.android.aconfig.test"
    name: "enabled_rw"
    state: ENABLED
    permission: READ_WRITE
}
```

The output Android.bp file will be like
```
VALUE_LIST_LIST = [
    aconfig-local-com.android.demoapp.test,
    aconfig-local-com.android.aconfig.test
]

aconfig_values {
    name: "aconfig-local-com.android.demoapp.test",
    package: "com.android.demoapp.test",
    srcs: [
        "override_values.textproto",
    ]
}

aconfig_values {
    name: "aconfig-local-com.android.aconfig.test",
    package: "com.android.aconfig.test",
    srcs: [
        "override_values.textproto",
    ]
}
```

Bug: 298692416
Test: source build/envsetup.sh && overrideflags, and then check the
output

Change-Id: Ife4063e079811e0b29046be7a6bc127cad668ed0

envsetup.sh

@@ -2054,6 +2054,11 @@ function avbtool() {
     "$ANDROID_SOONG_HOST_OUT"/bin/avbtool $@
 }
 
+function overrideflags() {
+    local T="$(gettop)"
+    (\cd "${T}" && build/make/tools/overrideflags.sh "$@")
+}
+
 validate_current_shell
 set_global_paths
 source_vendorsetup