Android: Allow custom compilers with NDK

If the user or toolchain file has already set `CMAKE_{C,CXX}_COMPILER`,
do not override it with the NDK-provided toolchain.

Fixes: #27162

Modules/Platform/Android-Determine-C.cmake

@@ -1,2 +1,5 @@
+if(CMAKE_C_COMPILER)
+  return()
+endif()
 include(Platform/Android/Determine-Compiler)
 __android_determine_compiler(C)

Modules/Platform/Android-Determine-CXX.cmake

@@ -1,2 +1,5 @@
+if(CMAKE_CXX_COMPILER)
+  return()
+endif()
 include(Platform/Android/Determine-Compiler)
 __android_determine_compiler(CXX)