To use google drive api, I have to play with the authentication using OAuth2.0. And I got a few question about this.
Client id and client secret are used to identify what my app is. But they must be hardcoded if it is a client application. So, everyone can decompile my app and extract them from source code. Does it mean that a bad app can pretend to be a good app by using the good app's client id and secret? So user would be showing a screen that asking for granting permission to a good app even though it is actually asked by a bad app? If yes, what should I do? Or actually I should not worry about this?
In mobile application, we can embedded a webview to our app. And it is easy to extract the password field in the webview because the app that asking for permission is actually a "browser". So, OAuth in mobile application does not have the benefit that client application has not access to the user credential of service provider?
I started writing a comment to your question but then found out there is too much to say so here are my views on the subject in the answer.
Yes there is a real possibility for this and there were some exploits based on this. Suggestion is not to keep the app secret in your app, there is even part in the spec that distributed apps should not use this token. Now you might ask, but XYZ requires it in order to work. In that case they are not implementing the spec properly and you should A not use that service (not likely) or B try to secure token using some obfuscating methods to make it harder to find or use your server as a proxy.
For example there were some bugs in Facebook library for Android where it was leaking tokens to Logs, you can find out more about it here http://attack-secure.com/all-your-facebook-access-tokens-are-belong-to-us and here https://www.youtube.com/watch?v=twyL7Uxe6sk. All in all be extra cautious of your usage of third party libraries (common sense actually but if token hijacking is your big concern add another extra to cautious).
I have been ranting about the point 2 for quite some time. I have even done some workarounds in my apps in order to modify the consent pages (for example changing zoom and design to fit the app) but there was nothing stopping me from reading values from fields inside the web view with username and password. Therefore I totally agree with your second point and find it a big "bug" in OAuth spec. Point being "App doesn't get access to users credentials" in the spec is just a dream and gives users false sense of security… Also I guess people are usually suspicions when app asks them for their Facebook, Twitter, Dropbox or other credentials. I doubt many ordinary people read OAuth spec and say "Now I am safe" but instead use common sense and generally not use apps they don't trust.