Add support for HXCPP_ANDROID_PLATFORM define

[tobil4sk]

Originally, android min sdk version could be configured using PLATFORM=android-xx. This was used because it was a sub directory of the android ndk: 9e716f1. Hxcpp used to extract the version from this define:

hxcpp/tools/hxcpp/Setup.hx

Lines 547 to 552 in 75108a5

	var platform = defines.get("PLATFORM");
	var id = Std.parseInt( platform.substr("android-".length) );
	if (id==0 || id==null)
	Log.error('Badly formed android PLATFORM "$platform" - should be like android-123');
	androidPlatform = id;
	Log.info("", "\x1b[33;1mUsing Android NDK platform: " + defines.get("PLATFORM") + "\x1b[0m");

However, for modern ndks, PLATFORM is no longer used to resolve any directory and since #825 it is not parsed by hxcpp for ndk 20 or newer either.

This left people with no way to configure the min sdk version. To give a way of doing this again, PLATFORM_NUMBER was made configurable in #992. However, given that it does not have a very descriptive name this seems like it was an internal variable that should not have been exposed like this. It has also never been documented.

This PR allows setting the minimum sdk version using HXCPP_ANDROID_PLATFORM. This is a flag that was already used internally by hxcpp and has a much more descriptive name.

I have therefore also deprecated the PLATFORM_NUMBER define with a warning.

[tobil4sk]

Actually, instead of hxcpp manually specifying which api version to use per ndk version, it should read the ndk's metadata to determine this, which avoids having to bump it every time and is closer to what it used to do with older ndks when it would read the platforms/ folder contents

[tobil4sk]

The default HXCPP_ANDROID_PLATFORM value is now set based on the ndk metadata, which means it won't be necessary in future to have PRs like: #1134 (or the platform version bump in #992).

Later it may be a good idea if the branch for ndks older than 20 would be unified with this (in terms of the logic at least), because right now we use the maximum version there, but the minimum version for newer ndks. However, we need to use the minimum platform version that supports the given architecture.