[Android] Make additional_apks option default to empty list.
[Android] Make additional_apks option default to empty list.

This option currently causes an error for tests run in platform
mode. Making additional_apks default to [] to fix this.

BUG=

Review URL: https://codereview.chromium.org/1376813002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#351342}

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