Using custom logger in Android SDK

[BartoszLitwiniuk]

Description

Added example how to replace default BacktraceLogger used for logging internal SDK information by custom implementation.

Motivation and Context

It's adding example how to use this feature for clients. Should be merged after merging PR which introducing this feature https://github.com/backtrace-labs/backtrace-android/pull/147.

Types of Changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)
• [x] Documentation fix (typos, incorrect content, missing content, etc.)