SCons: Build thirdparty code in own env, disable warnings

Also remove unnecessary `Export('env')` in other SCsubs,
Export should only be used when exporting *new* objects.

platform/android/SCsub

@@ -1,12 +1,12 @@
 #!/usr/bin/env python
 
+Import('env')
+
 import shutil
 from compat import open_utf8
 from distutils.version import LooseVersion
 from detect import get_ndk_version
 
-Import('env')
-
 android_files = [
 
     'os_android.cpp',
@@ -26,10 +26,6 @@ android_files = [
 #    'power_android.cpp'
 ]
 
-# env.Depends('#core/math/vector3.h', 'vector3_psp.h')
-
-#obj = env.SharedObject('godot_android.cpp')
-
 env_android = env.Clone()
 if env['target'] == "profile":
     env_android.Append(CPPFLAGS=['-DPROFILER_ENABLED'])
@@ -174,4 +170,4 @@ if lib_arch_dir != '':
     ndk_version = get_ndk_version(env["ANDROID_NDK_ROOT"])
     if ndk_version != None and LooseVersion(ndk_version) >= LooseVersion("15.0.4075724"):
         stl_lib_path = str(env['ANDROID_NDK_ROOT']) + '/sources/cxx-stl/llvm-libc++/libs/' + lib_arch_dir + '/libc++_shared.so'
-        env_android.Command(out_dir + '/libc++_shared.so', stl_lib_path, Copy("$TARGET", "$SOURCE"))
+        env_android.Command(out_dir + '/libc++_shared.so', stl_lib_path, Copy("$TARGET", "$SOURCE"))