MainActivity: allow amount for poo and pee events

An unspecified amount has also been added
to have the same semantics as before.

During these actions, the strings for title
and description of dialogs have been cleaned up.

app/src/main/res/values/arrays.xml

@@ -1,6 +1,7 @@
 <?xml version="1.0" encoding="utf-8"?>
 <resources>
     <string-array name="AmountLabels">
+        <item>@string/amount_unspecified</item>
         <item>@string/amount_little</item>
         <item>@string/amount_normal</item>
         <item>@string/amount_plenty</item>