Modernize android build and sample (#6229)

Android build was dated, using the Android.mk approach. Current
project configuration on Android encourages the usage of CMake, so we
are updating the android project as an example on how to use either the
Java/Kotlin generate code or the native C++ one.

android/app/src/main/cpp/flatbuffers/CMakeLists.txt

@@ -0,0 +1,59 @@
+# For more information about using CMake with Android Studio, read the
+# documentation: https://d.android.com/studio/projects/add-native-code.html
+
+# Sets the minimum version of CMake required to build the native library.
+
+cmake_minimum_required(VERSION 3.4.1)
+
+include_directories(${FLATBUFFERS_SRC}/include)
+
+set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=gnu++11 -fexceptions -Wall -DFLATBUFFERS_TRACK_VERIFIER_BUFFER_SIZE")
+
+# Certain platforms such as ARM do not use signed chars by default
+# which causes issues with certain bounds checks.
+set(CMAKE_CXX_FLAGS
+   "${CMAKE_CXX_FLAGS} -fsigned-char")
+
+set(FlatBuffers_Library_SRCS
+        ${FLATBUFFERS_SRC}/include/flatbuffers/base.h
+        ${FLATBUFFERS_SRC}/include/flatbuffers/flatbuffers.h
+        ${FLATBUFFERS_SRC}/include/flatbuffers/hash.h
+        ${FLATBUFFERS_SRC}/include/flatbuffers/idl.h
+        ${FLATBUFFERS_SRC}/include/flatbuffers/util.h
+        ${FLATBUFFERS_SRC}/include/flatbuffers/reflection.h
+        ${FLATBUFFERS_SRC}/include/flatbuffers/reflection_generated.h
+        ${FLATBUFFERS_SRC}/include/flatbuffers/stl_emulation.h
+        ${FLATBUFFERS_SRC}/include/flatbuffers/flexbuffers.h
+        ${FLATBUFFERS_SRC}/include/flatbuffers/registry.h
+        ${FLATBUFFERS_SRC}/include/flatbuffers/minireflect.h
+        ${FLATBUFFERS_SRC}/src/idl_parser.cpp
+        ${FLATBUFFERS_SRC}/src/idl_gen_text.cpp
+        ${FLATBUFFERS_SRC}/src/reflection.cpp
+        ${FLATBUFFERS_SRC}/src/util.cpp
+        ${FLATBUFFERS_SRC}/src/idl_gen_fbs.cpp
+        ${FLATBUFFERS_SRC}/src/code_generators.cpp
+        )
+
+set(FlatBuffers_Test_SRCS
+        ${FLATBUFFERS_SRC}/tests/test.cpp
+        ${FLATBUFFERS_SRC}/tests/test_assert.h
+        ${FLATBUFFERS_SRC}/tests/test_builder.h
+        ${FLATBUFFERS_SRC}/tests/test_assert.cpp
+        ${FLATBUFFERS_SRC}/tests/test_builder.cpp
+        ${FLATBUFFERS_SRC}/tests/native_type_test_impl.h
+        ${FLATBUFFERS_SRC}/tests/native_type_test_impl.cpp
+)
+
+add_library( # Sets the name of the library.
+             flatbuffers
+
+             ${FlatBuffers_Library_SRCS}
+             ${FlatBuffers_Test_SRCS}
+             ${Generated_SRCS}
+)
+
+add_library( # Sets the name of the library.
+             flatbuffers_tests
+
+             ${FlatBuffers_Test_SRCS}
+)