The default browser on Android is Google Chrome. This uses the Blink layout engine. For AOSP installations without the Google Apps, the default browser is the old "Browser" app that uses Webkit.
other third party browsers like Firefox uses Gecko, Opera uses Blink, Dolphin uses Webkit, and there are probably others. Additionally, also Samsung and HTC install different (non-Chrome) browsers on their phones. I do not know what they are, or what engine they use.
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