Bug 1360291 - Set minimum NDK platform version to 21 for 64-bit builds; r=nalexander

64-bit is only supported in android-21 and above in the NDK.

build/moz.configure/android-ndk.configure

@@ -14,7 +14,12 @@ js_option('--with-android-toolchain', nargs=1,
 js_option('--with-android-gnu-compiler-version', nargs=1,
           help='GNU compiler version to use')
 
-min_android_version = dependable(lambda: '9')
+@depends(target)
+def min_android_version(target):
+    if target.cpu in ['aarch64', 'x86_64', 'mips64']:
+        # 64-bit support was added in API 21.
+        return '21'
+    return '9'
 
 js_option('--with-android-version',
           nargs=1,