Besides AWS, so many SDKs are used client oriented design, like Dropbox and Facebook. The consequence of leaking those keys may not critical as the AWS though.
Correct. Even for the not rooted phones, attackers can send SMS, record audio or access SD card due to the permissions the target app applied. Also you may exploit some privilege escalation vulnerability on Android after you got a shell. It's phone-specific and app-specific though.
It's a vulnerability on Android Webview component, which supports a "addJavascriptInterface" method. This method allows you to call the Java native method by using a Javascript object inside the webpage. And, there is a trick that can bypass the restriction on classes that JSInterface object could access. You can call any method in any Java class. Such as Java.lang.Runtime.exec. You can google "addJavascriptInterface vulnerability". It's not a new vulnerability, but lots of app haven't fix it yet.