Add multiBackend Android flavor for runtime OpenGL/Vulkan switching (#4288)

platform/android/MapLibreAndroid/src/cpp/CMakeLists.txt

@@ -2,8 +2,18 @@ cmake_minimum_required(VERSION 3.10)
 
 project(MapLibreAndroid)
 
-add_subdirectory(../../../../../
-                 ../../../../../${ANDROID_ABI})
+# (a) Multi-flavor build is self-contained in MultiBackend.cmake and only entered for the
+# Gradle `multiBackend` flavor (which passes -DMLN_ANDROID_MULTI_BACKEND=ON). It never touches
+# the shared top-level CMakeLists; the single-backend path below is unaffected.
+if(MLN_ANDROID_MULTI_BACKEND)
+    include(${CMAKE_CURRENT_SOURCE_DIR}/MultiBackend.cmake)
+    return()
+endif()
+
+# (b) Build core into THIS configure's own build dir (was: ../../../../../${ANDROID_ABI}, an
+# out-of-tree path both multi sub-builds collided on). Each configure now has its own dir, so
+# the multi sub-builds isolate automatically -- no MLN_ANDROID_TOP_BINARY_DIR guard needed.
+add_subdirectory(../../../../../ ${CMAKE_CURRENT_BINARY_DIR}/maplibre-core)
 
 set(CMAKE_CXX_STANDARD 20)
 set(CMAKE_CXX_STANDARD_REQUIRED ON)