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/build.gradle.kts

@@ -33,7 +33,8 @@ android {
         minSdk = 31
         targetSdk = 37
         versionCode = 8
-        versionName = "0.2.00-alpha"
+        // -PgearslipVersionName=0.1.99-test makes a build that thinks it's older, to try the updater.
+        versionName = project.findProperty("gearslipVersionName")?.toString() ?: "0.2.00-alpha"
         manifestPlaceholders["appLabel"] = "Gearslip"
         // Where opted-in phones send their daily usage note: the stats-worker/ Worker's address,
         // set as gearslipStatsUrl in gradle.properties. Left empty, nothing is ever sent.