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: refs/heads/master@{#359114}
1 file changed