fix(samples): Add Retrofit R8 full-mode keep rules to Android sample (#5727)

The Android sample's release build uses R8 full mode, which strips generic signatures from classes that aren't kept. This broke Retrofit call-adapter creation for GithubAPI, crashing SecondActivity in onResume with 'Unable to create call adapter for interface retrofit2.Call'.

Add the standard Retrofit R8 keep rules (Signature attributes, retrofit2.Call/Response, service interfaces) and keep the Repo response model for Gson.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>