x/mobile: Permissions are needed in AndroidManifest.xml file

[ghost]

Many native Golang features are not usable without specifying permissions in AndroidManifest.xml. Suggest to put permissions by the time each package is imported, for example:

add the following permission when package "os" is imported:
< uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" / >

and add the following permission when package "net" is imported:
< uses-permission android:name="android.permission.INTERNET" / >

[ghost]

A good reference: http://stackoverflow.com/questions/33815541/golang-gomobile-app-cannot-generate-files/33819669#33819669

[rakyll]

I proposed the idea of automatically adding android.permission.INTERNET if net package is imported a while ago but it is considered too aggressive. There is no easy way to determine if the application is going to make an external connection.

The same rationale applies to the os package. Maybe it is easier to mark storage related functions in the scope of os.

/cc @crawshaw @hyangah