Introduce UUIDMap for storing objects that are keyed by a UUID

This patch introduces the new ``UUIDMap`` type, reducing code duplication and
making UUID lookups faster. We currently already use UUIDs as the identifier for
the ``DatabaseEntry`` and ``Slot`` types, but the way lookups by UUID work are
kind of ugly, as we simply iterate over the list until we find a match. As we're
probably going to have more types like this soon (groups and icons, for
example), I figured it'd be good to abstract this away into a separate type and
make it a map instead of a list.

The only thing that has gotten slower is the ``swap`` method. The internal
``LinkedHashMap`` retains insertion order with a linked list, but does not know
about the position of the values, so we basically have to copy the entire map to
simply swap two values. I don't think it's too big of a deal, because swap
operations still take less than a millisecond even with large vaults, but
suggestions for improving this are welcome.

I had to update gradle and JUnit to be able to use the new ``assertThrows``
assertion method, so this patch includes that as well.

build.gradle

@@ -6,7 +6,7 @@ buildscript {
         google()
     }
     dependencies {
-        classpath 'com.android.tools.build:gradle:3.3.2'
+        classpath 'com.android.tools.build:gradle:3.4.1'
 
         // NOTE: Do not place your application dependencies here; they belong
         // in the individual module build.gradle files