drm_hwcomposer: Make platform selection based on weak/strong symbols.

Current OOP based selection give no benefit since we are selecting
only single platform at compile time and have no plans to
instantiate and control multiple platforms on the same system.

We also can't use definition-based enabling/disabling generic
planners, importers due to file-groups usage in Android.bp
for Imagination and Hikey platforms.

This commit removes code duplication in planner and platform instantiation code.
We also do not need platform-specific header files anymore, so remove them.

Signed-off-by: Roman Stratiienko <roman.stratiienko@globallogic.com>