tools/battor_agent: Removes CrOS and Android from target platforms

https://crrev.com/1611533002/ added necessary serial device enumeration
to the agent, which broke the Android build because I was inadvertently
building the agent on Android builders.

My goal is to have battor_agent only be buildable on Mac, Windows, and
Linux desktop machines. If I did that wrong, please let me know!

Review URL: https://codereview.chromium.org/1611353003

Cr-Commit-Position: refs/heads/master@{#371367}

• BUILD.gn[diff]
• build/gn_migration.gypi[diff]