libsnapshot: Use a real fake super partition for tests.

We are currently creating test partitions via ImageManager, which
unfortunately leads to a lot of hacks in my local tree to test
first-stage init. ImageManager devices look nothing like real partitions
in super, and it's a lot of work to massage it to act the same.

Instead, let's create an actual super partition. We can do this via
ImageManager, and give it its very own partition table. To make this
work, we need a special IPartitionOpener that will redirect requests for
"super" to our temporary device. A new .cpp file has been added to house
this. A few other tidbits have moved there as well.

This makes setup code a bit more complicated, but now our tests will
behave much closer to a real device.

Bug: 139204329
Test: libsnapshot_test gtest
Change-Id: I2f40109cd3c1d6343d6f52979789e25f9afd533a