[android] Remove Fabric and Crashlytics on Android (expo#8211)

# Why

Closes expo#8047 

# How

I removed:
- Fabric and Crashlytics setup
- `FABRIC_API_KEY` and `FABRIC_SECRET_KEY` from secrets keys
- `fabric.properties` files
- Removed Fabric and Crashlytics files from cluttered `.gitignore` files

# Test Plan

CI jobs passed, Android's client builds and works as expected.

android/.gitignore

@@ -152,12 +152,6 @@ atlassian-ide-plugin.xml
 # Mongo Explorer plugin
 .idea/mongoSettings.xml
 
-# Crashlytics plugin (for Android Studio and IntelliJ)
-com_crashlytics_export_strings.xml
-crashlytics.properties
-crashlytics-build.properties
-fabric.properties
-
 ### AndroidStudio Patch ###
 
 !/gradle/wrapper/gradle-wrapper.jar
@@ -205,8 +199,6 @@ cmake-build-debug/
 # Ruby plugin and RubyMine
 /.rakeTasks
 
-# Crashlytics plugin (for Android Studio and IntelliJ)
-
 ### Intellij Patch ###
 # Comment Reason: https://github.com/joeblau/gitignore.io/issues/186#issuecomment-215987721