Fetch traces async

Due to the large amount of traces on flicker, the tests take 15min+ to copy the files from the device to the host after the test execution. Instead, start copying the files asynchronously.

It is not possible to use the async mechanism from Tradefed's FilePullerLogCollector because it either: (1) pushes the files form the directory multiple times -- once per test -- creating a lot of duplicates; or (2) cleans up the directory without checking for new files, losing some files.

This CL creates an asynchronous file upload mechanism for flicker using coroutines

Bug: 216111397
Test: atest FlickerTests WMShellFlickerTests
Change-Id: I5bbae834cf3439950626823c6b665c733e27e517