vulkan: fix glibc AOSP build

Certain AOSP targets feature a glibc or musl-based build,
where (__ANDROID__) isn't defined, but:

- ANDROID is defined
- the relevant headers are present.

For such builds, buffer_handle_t shouldn't be defined
internally.

Reviewed-by: Roman Stratiienko <r.stratiienko@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/31030>

include/vulkan/vk_android_native_buffer.h

@@ -1,5 +1,23 @@
-/* MESA: A hack to avoid #ifdefs in driver code. */
-#ifdef __ANDROID__
+/*
+ * MESA: buffer_handle_t is defined by all Mesa builds, even if
+ * one is building for a non-Android target.  This avoids unnecessary
+ * conditionals in driver code.
+ *
+ * We don't need to define buffer_handle_t locally when (__ANDROID__)
+ * or ANDROID are set.  Here's the distinction between the two:
+ *
+ * - AOSP always defines ANDROID, since it just means one is using the
+ *   AOSP tree. It means the build environment is Android, roughly.
+ * - __ANDROID__ is defined by the toolchain.  This typically means the
+ *   build target is Android.
+ *
+ * If the build environment is Android, AOSP can provide common Android
+ * headers, such as <cutils/native_handle.h>.  This allows one to build
+ * and test certain aspects of Android window system code, on the host
+ * system rather the build target.
+ */
+
+#if defined(__ANDROID__) || defined(ANDROID)
 
 #include <cutils/native_handle.h>
 #if ANDROID_API_LEVEL < 28