android build: fix warning re ndk_api

"NDK API target was not set manually, using the default of 21 = min(android-api=28, default ndk-api=21)"

electrum/gui/kivy/tools/buildozer.spec

@@ -57,7 +57,7 @@ android.permissions = INTERNET, CAMERA
 # (int) Android API to use
 android.api = 28
 
-# (int) Minimum API required (8 = Android 2.2 devices)
+# (int) Minimum API required. You will need to set the android.ndk_api to be as low as this value.
 android.minapi = 21
 
 # (int) Android SDK version to use
@@ -66,6 +66,9 @@ android.sdk = 24
 # (str) Android NDK version to use
 android.ndk = 14b
 
+(int) Android NDK API to use (optional). This is the minimum API your app will support.
+android.ndk_api = 21
+
 # (bool) Use --private data storage (True) or --dir public storage (False)
 android.private_storage = True