Migrate off Unsafe (guava-android)

[cpovirk]

This is the next step now that we've migrated guava-jre off (#6806).

Some recent notes on Android: #6806 (comment)

Part of the work here is to have a fallback at all for a case or two that is missing one.

Then, ideally, we'd avoid even trying to use Unsafe in cases where it would produce a warning. But we don't want to regress performance on Android or ideally even the JVM.

One other small note: Currently, the Android flavor has no module-info. Today, if we were to give it one, we should really have a "full" requires line for jdk.unsupported. After this issue, though, we could mirror the JRE variant's static line.

[cpovirk]

I posted some thoughts in https://issuetracker.google.com/issues/134377851#comment3.