Dynamic versions (using +) of dependencies in gradle
I am using compile 'com.localytics.android:library:3.8+' in my project.
I want to know that is the latest version of this dependency is picked when the release apk is created or every time when the user opens the app (downloaded from playstore).
new version downloads when You create a new build. There is nothing changes in it before You create a new one.
+ here known as dynamic dependencies.
Simply + will use the latest or updated dependency but Using dynamic dependencies (for example, com.localytics.android:library:3.8+ ) can cause unexpected version updates and regression incompatibilities as suggested by google docs
Pros
Cons
Can break your current code flow due to changes in lib code
Usually there can be some bug found in the new releases so be ready to face unexpected behavior or crash
Conclusion : As advised by google always specify the library version check under Adding Support Libraries
You should avoid the use of the + in your dependencies.
In this way you will not be able to replicate your build in the future because you can't know the versions used in your app.
Dynamic versions add nondeterminism to your build and can introduce unexpected behavior changes to your app.
To check Newer Library Versions Available you can use a lint check:
Go to Settings > Editor > Inspections > "Newer Library Versions Available"
Enable the Newer Library Versions Available inspection
Go to Analyze > "Run Inspection By Name" and search for Newer Library Versions Available 
You will launch a link check
