Add tags to treat variability inside the files, in a fine granularity way.

In XML files we use:

<!-- FEATURE BEGIN A_FEATURE -->

your XML code that treat the variability

<!-- FEATURE END A_FEATURE -->

In Java files we use:

//FEATURE BEGIN A_FEATURE

your Java code that treat the variability

//FEATURE END A_FEATURE

Where A_FEATURE must be a *value* constant that represents a feature,
defined in configspl.py file.
Treat PLAYLIST feature in AndroidManifest.xml

The CORE feature is a fake feature, just "clean" the file until we dont
treat the other features