base: Bump default android version to 6.0.99
base: Bump default android version to 6.0.99

The Android Marshmallow release is 6.0.

Review URL: https://codereview.chromium.org/1395633004

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#354042}

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