gitignore ReactAndroid prebuilt libs (#27487)

Summary:
This PR changes gitignore to ignore prebuilt libs in ReactAndroid as a whole, because we added 64 bit platforms and their files are showing up as changes.

## Changelog

[GENERAL] [Changed] - gitignore ReactAndroid prebuilt libs
Pull Request resolved: #27487

Test Plan: After ReactAndroid build, you won't see prebuilt lib changes in git.

Differential Revision: D18992774

Pulled By: hramos

fbshipit-source-id: 33d3e01f59945dbc18ae4d9e3e3ac4b0e5b5e764

.gitignore

@@ -32,8 +32,7 @@ project.xcworkspace
 # Buck
 .buckd
 buck-out
-/ReactAndroid/src/main/jni/prebuilt/lib/armeabi-v7a/
-/ReactAndroid/src/main/jni/prebuilt/lib/x86/
+/ReactAndroid/src/main/jni/prebuilt/lib/
 /ReactAndroid/src/main/gen
 
 # Watchman