How do I fully uninstall the Bitcoin Wallet app for Android?
The size of the blockchain download by BW has significantly reduced recently, and is only headers at best so you're only looking for a 20-40Mb file not gigabytes.

You should be able to locate the private keys here:

/data/data/de.schildbach.wallet/files


If you need assistance with accessing your rooted phone (you probably don't) I wrote an article detailing Bitcoin recovery operations on Android that may provide useful supporting information.