xrt: use different values for XRT_MAX_LAYERS on Android

Part-of: <https://gitlab.freedesktop.org/monado/monado/-/merge_requests/2639>

src/xrt/include/xrt/xrt_limits.h

···
 /*!
  * Max number of layers which can be handled at once.
  */
+#ifdef XRT_OS_ANDROID
+#define XRT_MAX_LAYERS 32
+#else
 #define XRT_MAX_LAYERS 128
+#endif
 
 /*!
  * @}