First, sorry for my english, i'm brazilian.
I don't know if it will help anyone, but I found a way to the userMedia work on android on phonegap build.
I searched here in scirra forum, but have not found a solution.
So, after reading the PhoneGap Documentation and based on other android apps, I found a solution (or not).
First, on construct 2, before exporting the project to Phonegap you should check the "use camera".
After you export to phonegap, and generated the .apk, you must edit the AndroidManifest.xml (for this you will need an apk editor, like Apk Manager... I used the app "apk editor pro" on my device.)
On AndoidManifest.xml you need to add the permission to android use the camera, for this, add the line:
<uses-permission android:name="android.permission.CAMERA" />[/code:37j0qrzf]
And Save. Now, just install the modified .apk, and run. It Works for me. Enjoy!!!
Any issue just ask. Thanks!