Fix/android toolchain c library (#11586)

* Do not crush if the conanfile is a txt

* Fix CMakeToolchain for Android with pure C library

conan/tools/cmake/toolchain/blocks.py

@@ -286,7 +286,9 @@ class AndroidSystemBlock(Block):
     template = textwrap.dedent("""
         # New toolchain things
         set(ANDROID_PLATFORM {{ android_platform }})
+        {% if android_stl %}
         set(ANDROID_STL {{ android_stl }})
+        {% endif %}
         set(ANDROID_ABI {{ android_abi }})
         include({{ android_ndk_path }}/build/cmake/android.toolchain.cmake)
         """)
@@ -303,7 +305,7 @@ def context(self):
 
         # TODO: only 'c++_shared' y 'c++_static' supported?
         #  https://developer.android.com/ndk/guides/cpp-support
-        libcxx_str = str(self._conanfile.settings.compiler.libcxx)
+        libcxx_str = self._conanfile.settings.get_safe("compiler.libcxx")
 
         android_ndk_path = self._conanfile.conf.get("tools.android:ndk_path")
         if not android_ndk_path:

conans/test/integration/toolchains/cmake/test_cmaketoolchain.py

@@ -473,3 +473,20 @@ def generate(self):
     assert cache_variables["CMAKE_SH"] == "THIS VALUE HAS PRIORITY"
     assert cache_variables["CMAKE_POLICY_DEFAULT_CMP0091"] == "THIS VALUE HAS PRIORITY"
     assert cache_variables["CMAKE_MAKE_PROGRAM"] == "MyMake"
+
+
+def test_android_c_library():
+    client = TestClient()
+    conanfile = textwrap.dedent("""
+        from conans import ConanFile
+
+        class Conan(ConanFile):
+            settings = "os", "arch", "compiler", "build_type"
+            generators = "CMakeToolchain"
+
+            def configure(self):
+                del self.settings.compiler.libcxx
+
+        """)
+    client.save({"conanfile.py": conanfile})
+    client.run("create . foo/1.0@ -s os=Android -s os.api_level=23 -c tools.android:ndk_path=/foo")