Remove flaky from default excluded tests

Atest cannot override include-annotation and exclude-annotation from the Android.xml, only append new ones.

Ideally we should have a single test configuration for all flicker tests (presubmit/postsubmit/flaky) and adjust the arguments according to the type of test we intend to run.

Presubmit:
    atest FlickerTests -- --module-arg FlickerTests:include-annotation:android.platform.test.annotations.Presubmit --module-arg FlickerTests:exclude-annotation:androidx.test.filters.FlakyTest

Postsubmit:
    atest FlickerTests -- --module-arg FlickerTests:include-annotation:android.platform.test.annotations.Postsubmit --module-arg FlickerTests:exclude-annotation:androidx.test.filters.FlakyTest

Flaky:
    atest FlickerTests -- --module-arg FlickerTests:include-annotation:androidx.test.filters.FlakyTest

To run WMShellFlickerTests, just replace FlickerTests for WMShellFlickerTests in the commands above.

Bug: 170490377
Test: atest FlickerTests WMShellFlickerTests
Change-Id: Ic252f27065714e9039e3c0f33659b98eb288aa6f