HID: hid-input: occasionally report stylus battery even if not changed

There are styluses that only report their battery status when they are
touching the touchscreen; additionally we currently suppress battery
reports if capacity has not changed. To help userspace recognize how long
ago the device reported battery status, let's send the change event through
if either capacity has changed, or at least 30 seconds have passed since
last report we've let through.

Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>