Improve Android .gitignore comments

Android.gitignore

-# built application files
+# Built application files
 *.apk
 *.ap_
 
-# files for the dex VM
+# Files for the Dalvik VM
 *.dex
 
 # Java class files
 *.class
 
-# generated files
+# Generated files
 bin/
 gen/
 
-# Ignore gradle files
+# Gradle files
 .gradle/
 build/