Move flaky to single test level instead of class level

This will promote more tests into pre-submit. The standard is, if a test
is shown stable in the past 7 days, the flaky is removed.

Some clean-up also applied.

Bug: 123654474
Test: pre-submit

Change-Id: I4d0563eae9e3e7db4e6cffb2630c5930865ae1aa