Update the test reference facility to use Python enums

Use Python enum instead of the custom `AtestEnum` enumeration class now
that Atest requires Python 3.4+.

Note that related data that was indexed by the enum name in separate
dictionaries is now moved into enum attributes. This keeps related code
together and makes it easier to access.

Bug: 218441706
Test: rm $ANDROID_HOST_OUT/indexes/*idx
Test: atest-dev --dry-run PacketFragmenterTest -vc
Test: atest-dev --dry-run -cv libcore.java.time.chrono.ChronologyDisplayNameTest#testEras
Test: atest-dev --dry-run -cv com.android.server.wm
Test: atest-dev PacketFragmenterTest -vc
Test: atest-dev -cv libcore.java.time.chrono.ChronologyDisplayNameTest#testEras
Test: atest-dev -cv com.android.server.wm
Change-Id: Ibedd2d85c209703bbc6722a844d562c495aaf015