TIF: add PreviewPrograms

Currently, Programs class is being used for storing both linear
tv program and preview program. This change adds PreviewPrograms
class for clear separation.

Test: build and cts-tradefed run cts -m CtsTvTestCases
Bug: 34853064
Change-Id: I9789e0296941783c6096d1ff9a0d81e11370ab65