add strict, tested validation of Android/F-Droid package names

Android has stricter rules than Java for Package Names, but anything the
Python regex thinks is valid must be valid according to Java's rules too.

https://developer.android.com/studio/build/application-id

tests/valid-package-names/test.py

@@ -0,0 +1,32 @@
+#!/usr/bin/env python3
+
+import re
+
+
+def test(packageName):
+    m = ANDROID_APPLICATION_ID_REGEX.match(packageName.strip())
+    return m is not None
+
+
+ANDROID_APPLICATION_ID_REGEX = re.compile(r'''(?:^[a-z_]+(?:\d*[a-zA-Z_]*)*)(?:\.[a-z_]+(?:\d*[a-zA-Z_]*)*)*$''')
+valid = 0
+invalid = 0
+
+test('org.fdroid.fdroid')
+with open('valid.txt', encoding="utf-8") as fp:
+    for packageName in fp:
+        packageName = packageName.strip()
+        if not test(packageName):
+            valid += 1
+            # print('should be valid:', packageName)
+
+with open('invalid.txt', encoding="utf-8") as fp:
+    for packageName in fp:
+        packageName = packageName.strip()
+        if test(packageName):
+            invalid += 1
+            print('should be not valid: "' + packageName + '"')
+
+
+print(valid, 'Java thinks is valid, but the Android regex does not')
+print(invalid, 'invalid mistakes')