Update Android test instructions.
Update Android test instructions.

- Update emulator API level to 23 to match build/common.gypi.
- Remind readers to build for x86 when using the x86 emulator.

Review URL: https://codereview.chromium.org/1433213002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#359114}

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