fix tests to work when only Debian Android SDK is installed

jenkins.debian.net only had `apt install android-sdk`, it does not have any
of Google's packages installed.

tests/common.TestCase

@@ -70,7 +70,10 @@ class CommonTest(unittest.TestCase):
             return False
 
     def _find_all(self):
-        for cmd in ('aapt', 'adb', 'android', 'zipalign'):
+        tools = ['aapt', 'adb', 'zipalign']
+        if os.path.exists(os.path.join(os.getenv('ANDROID_HOME'), 'tools', 'android')):
+            tools.append('android')
+        for cmd in tools:
             path = fdroidserver.common.find_sdk_tools_cmd(cmd)
             if path is not None:
                 self.assertTrue(os.path.exists(path))