Soong: Use ctx.Writer in flag usage for dumpVar and dumpVars in soong_ui.

ctx.Writer is recommended stream to use for standard io.

Fixes: b/135487952
Test: Lunch and executed ./out/soong_ui --dumpvar-mode and
--dumpvars-mode to check the usage is output to the console.

Change-Id: I38351ebdb05f49f92d6b4bfb0423092299d1c5ea