[google_sign_in] Adopt task queues for Android

[stuartmorgan-g]

Replaces manual offloading of some method implementations to a
background task queue with just using the new method channel task queue
system to have those methods be called on a background queue
automatically.

This simplifies the code, and also allows removing the Guava dependency with
a couple of other minor changes:

• Replaces Joiner with Java 8's String.join.
• Replaces Strings.isEmptyOrNull with a local helper method.

Fixes flutter/flutter#13991

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[stuartmorgan-g]

Some of this code may go away anyway in the conversion to Credential Manager, but I wanted to split out this cleanup to simplify future changes.

[stuartmorgan-g]

test-exempt: code refactor with no semantic change

[camsim99]

LGTM!