Reduce code duplication

[Marcono1234]

Resolves some of the issues with google#2864, mainly:

• reduces code duplication; GsonBuilderHelper is not creating the list of factories anymore
• fixes the test issues

See also my review comments below for additional explanations for the changes.
Feel free to adjust the changes here in case you don't want to directly integrate them as is into your PR.

Generally I really like your approach though!

[Marcono1234]

Have reverted this to a HashMap because the EnumMap was causing issues for the test-shrinker integration tests; the JDK was unable to create the EnumMap for the enum

[MukjepScarlet]

What issue does EnumMap have? I'm not sure because we have the classOfT here

[Marcono1234]

Run mvn clean verify --projects test-shrinker --also-make, it will fail with the following error when using EnumMap:

...
Caused by: java.lang.RuntimeException: Test failed: Write: Enum
        at com.example.ai.a(Unknown Source)
        at com.example.Main.runTests(Unknown Source)
        at java.base/jdk.internal.reflect.DirectMethodHandleAccessor.invoke(DirectMethodHandleAccessor.java:103)
        ... 39 more
Caused by: java.lang.NullPointerException: Cannot read the array length because "this.keyUniverse" is null
        at java.base/java.util.EnumMap.<init>(EnumMap.java:139)
        at com.a.a.b.a.i.<init>(Unknown Source)
        at com.a.a.b.a.i.<init>(Unknown Source)
...

I think the issue is that ProGuard and R8 shrink the enum class so much that the JDK cannot obtain the enum constants. This is also why EnumTypeAdapter uses class#getDeclaredFields() and isEnumConstant() instead of simply Class#getEnumConstants().