Update sample app readme

[mingxiajiang-okta]

Description:

Testing details:

• [ ] Verified basic functionality of change
• [ ] Added tests

Other considerations:

• [ ] Has security implications
• [ ] Has UX changes
• [ ] Version bump required
• [ ] Followed recommendations from the Android Secure Coding Guidelines

RESOLVES:

OKTA-517954

Primary Reviewer(s): @FeiChen-okta @buntyjoshi-okta

Additional Reviewers:

Security Reviewer(s) (@ okta/rex-team if necessary):

UX Reviewer(s) (if necessary):