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 the OEM FirstVoices Android app, this updates the KMAPro `build.sh` script to download the latest default language resources (keyboard and dictionary) as part of the build process.
n trying to build Keyman for Android on Linux, the following issues needed to be fixed:
Set `build.sh` and `gradlew` scripts to executable
KMEA/build.sh needs an OS check so build.bat is only called on Windows
web/source/build.sh needs $compiler and $copmiler_warnings values in quotes
