android: fails to build

[phlip9]

Related PR: #125

Description

Building on Android fails:

$ cargo check --target=aarch64-linux-android
    Checking keyring v2.0.2
error[E0433]: failed to resolve: use of undeclared crate or module `default`
   --> src/lib.rs:176:54
    |
176 |         static ref DEFAULT: Box<CredentialBuilder> = default::default_credential_builder();
    |                                                      ^^^^^^^
    |                                                      |
    |                                                      use of undeclared crate or module `default`
    |                                                      help: a struct with a similar name exists: `DEFAULT`

error: could not compile `keyring` (lib) due to previous error

This is because there is no default keyring backend for the platform, even though a user could provide their own backend.