Fixes up scripts (except under /linux) to use `#!/usr/bin/env bash`
instead of `#!/bin/bash` or `#!/bin/sh` so that we don't end up with
the ancient version of bash supplied with macOS.
This became urgent with this PR, because of bash-4.xisms in
build-utils.sh, for example on line 572:
```
if [[ -v _builder_params[$e] ]]; then
```
Similar to #3412
1. Adds a similar script from Windows to identify unused Android strings (from KMAPro and KMEA strings.xml)
2. Remove the unused strings from strings.xml
As part of the manual cleanup, consolidated the strings `missing_metadata` and `invalid_metadata`.
Also changed string name `model` to a more specific `model_label`
