fix: turn the screen on when an alarm or timer goes off

SystemUI launches a full-screen intent only when a notification is first
posted, never on an update. The alarm service went foreground with a
placeholder that had none and added it in the update, so a locked phone
could ring behind a black screen. The placeholder now carries it.

Timers had no full-screen screen at all. A finished timer now opens
TimerRingActivity over the lock screen with Stop and +1 min, posted
under a second notification id so the expiry is a fresh post. It only
does so while the timer is sounding, so it never covers a ringing
alarm's screen or wakes the display for an already auto-silenced timer.

Fixes #15

app/src/main/AndroidManifest.xml

@@ -183,6 +183,18 @@
             android:taskAffinity=""
             android:turnScreenOn="true" />
 
+        <!-- The timer's ring screen: the alarm's window settings, opened by the
+             expired-timer notification's full-screen intent (issue #15). -->
+        <activity
+            android:name=".ui.ring.TimerRingActivity"
+            android:exported="false"
+            android:excludeFromRecents="true"
+            android:launchMode="singleTask"
+            android:showOnLockScreen="true"
+            android:showWhenLocked="true"
+            android:taskAffinity=""
+            android:turnScreenOn="true" />
+
         <!-- The ring itself. systemExempted per D17: not mediaPlayback (an alarm
              is not the user's media session) and emphatically not shortService,
              which caps at ~3 minutes against a 10-minute ring window. -->