build: Disable Android debug variants outside the sample app (#5825)

Unit tests already target the release variant (testBuildType = release)
and CI disabled the debug variant. Disable the debug variant locally too
so local builds match CI and skip building the unused variant. The
sample app keeps both variants for manual runs and profiling.

Update the contributor docs and test skill to reference
testReleaseUnitTest instead of testDebugUnitTest.

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>