Fix filamat build (#1005)

Filamat public headers were including a private header. This PR fixes
this problem. It also forces filamat to always be compiled to avoid
breaking filamat without noticing. The flag `-l` is not available
anymore in build.sh as a result.

android/samples/README.md

@@ -44,9 +44,6 @@ Demonstrates how to render into a `TextureView` instead of a `SurfaceView`:
 Demonstrates how to programatically generate Filament materials, as opposed to compiling them on the
 host machine:
 
-**Important:** The `material-builder` sample requires the `filamat` static library which can be compiled by adding
-the `-l` flag to the `./build.sh` command.
-
 ![Material Builder](../../docs/images/samples/sample_image_based_lighting.jpg)
 
 ## Prerequisites

android/samples/material-builder/README.md

@@ -14,11 +14,10 @@ library is slower and thus it is not built by the default `build.sh` command. To
 flag to the `build.sh` command:
 
 ```
-./build.sh -i -l -p android release
+./build.sh -i -p android release
 ```
 
 Explanation of flags:
 
 - The `-i` flag installs the libraries to `out/android-release/`, so this sample can find them.
-- The `-l` flag additionally builds the `filamat` library, needed by the `material-builder` sample.
 - The `-p` flag and `android` argument choose Android as the platform to build for.