Add @FlakyTest annotation to @Postsubmit tests

@Postsubmit may be used for something else in the future, so replace the
annotation with the more correct and specific annotation @FlakyTest

Bug: 35710737
Test: No change to behavior
Change-Id: I740c2c9e3cfcd78481d4a6f8f179872cd7832beb
Merged-In: I740c2c9e3cfcd78481d4a6f8f179872cd7832beb
2 files changed