fix: allow 'reboot' to application when on on battery

Having to press the power button again to wake up the device is less
annoying to the user than not being able to apply settings when the
device is on battery.

Signed-off-by: Matthias Blankertz <matthias@blankertz.org>

software/tests/test_playerapp.py

@@ -139,6 +139,9 @@ class FakeConfig:
     def get_tagmode(self):
         return 'tagremains'
 
+    def get_volume_max(self):
+        return 255
+
 
 def fake_open(filename, mode):
     return FakeFile(filename, mode)