update checker: finds new releases, downloads and installs them

Checks GitHub's latest release once a day when the app opens and after a
drive. A new version shows as a card on Home, a notification after the
drive, and a card on the car screen at plug-in while parked. Downloads are
checked against GitHub's SHA-256 and never install while the car is
connected. Update notifications can be turned off in Settings.

gearslip/src/main/res/xml/share_paths.xml

@@ -1,5 +1,6 @@
 <?xml version="1.0" encoding="utf-8"?>
 <paths>
-    <!-- Only the log copies made for sharing; nothing else in the app is exposed. -->
+    <!-- Only log copies made for sharing and downloaded updates; nothing else in the app is exposed. -->
     <cache-path name="shared_logs" path="shared/" />
+    <cache-path name="updates" path="updates/" />
 </paths>