Improve the reliability of cronet_http/ok_http workflows

[brianquinlan]

Make the package:cronet_http CI run more reliably by:

• running it on a better machine
• increasing the timeout to 30 minutes
• don't remove unnecessary tools (takes about 4 minutes, increasing the likelyhood of a timeout)
• enable the KVM for the android emulator
• don't try to use Google Play services with Android API level 21

Make the package:ok_http CI run more reliably by:

• correctly identifying the name of the workflow files
• running it on a better machine
• enable the KVM for the android emulator

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[github-actions]

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pkgs/http/example/main.dart