[Composer] Fix compiler warnings

When included by SurfaceFlinger, there were various warnings in the
ComposerCommandBuffer headers for conversions with -Wconversion enabled
that necessitated disabling that warning for these headers. These have
been fixed by adding explicit static_casts to the needed integer type.

In addition "#warn" was not a valid preprocessor directive, as used at
the top of each file if LOG_TAG is not defined. This was replaced by a
correct "#warning" directive.

Finally, there were also some unused includes reported by in-editor
clangd checks. Those have been removed.

Flag: EXEMPT build and lint warnings only
Bug: None
Test: Builds

Change-Id: I4918a115fab76c9cebee1f9fcfbb841e6cef6690