Add heatmap-color support on Android

[tobrun]

Capturing from @mourner in #11046 (comment):

I disabled any code generation for heatmap-color on the Android side because it requires being able to pass expressions as its values — as far as I remember, we had some bits missing about expressions which made it very hard to make it work.

Ticketing this out so we don't forget adding this for the final release.

[tobrun]

I reenabled generating the heatmap-color property from the specification and I'm hitting:

update: the reason why the system can't handle automatic conversion:

// from paint_property.hpp

/*
 * Special-case paint property traits for heatmap-color, needed because
 * heatmap-color values do not fit into the
 * Undefined | Value | {Camera,Source,Composite}Function taxonomy that applies
 * to all other paint properties.
 *
 * These traits are provided here--despite the fact that heatmap-color
 * is not used like other paint properties--to allow the parameter-pack-based
 * batch evaluation of paint properties to compile properly.
 */

we need a conversion to support this special property.

[mourner]

@tobrun I recall that we were not able to convert properly at the time of the PR, but @anandthakker @ChrisLoer might be working on this now, or at least expect to land some improvements soon — can you comment?

[ChrisLoer]

@mourner we're close to landing an Expression::serialize() that returns an mbgl::Value is that what you'd need?

[tobrun]

@ChrisLoer awesome to hear, that would make integration from my end a lot easier

[anandthakker]

@tobrun yeah, I think this needs the equivalent of #11074

[tobrun]

fixed with #11220, getter will be added with #11352