STL: Add ProgressConverter class [1/2]

This new class will make it easier to create new progress value
converters, and it will provide better support from the IDE during
autocomplete.

Added two functions (linear and tanh) to generate these converter and a
default value.

Test: Just a refactor
Bug: 336710600
Flag: com.android.systemui.scene_container
Change-Id: If72a6d31de2709680ec596e36e526b9bb06e0c6f