Add developer wallet features to fakewallet

[Michaelsulistio]

Context

fakewallet can be improved upon to make it the go-to Android wallet for developer testing MWA. This issue aims to track feature requests and ideas that can make fakewallet a more fleshed out developer wallet.

Motivations

• fakewallet loads much faster than a Phantom/Solflare which are prone to lag and crashing when using MWA on an emulator.

Ideas

• Persistent keypair storage so the wallet can be used consistently.
• Enable a way to import an existing keypair. Gradle settings or in-app import.
• Option to toggle fingerprinting/authentication
• Option to enable hot wallet mode and no authentication needed
• Transaction inspector UI during signing
  • Displays what transaction looks like, accounts, signers, what's going to be signed, etc
• An APK download link for easy installation, rather than cloning this repo and building from Android Studio