TEST: Fix host native test script

TEST: Fix host native test script

* Use build/soong/soong_ui.bash to get needed environment variables
  instead of getting them directly from the parent shell
* Simplify the build command use to 'soong_ui.bash' directly so that
  we can remove the call to envsetup
* When ANDROID_BUILD_TOP is not set, try finding it by looking for
  build/soong/soong_ui.bash in all parent directories until found,
  and get TOP variable from it
* Similarly, for ANDROID_HOST_OUT, use the same soong_ui.bash script to
  get it, instead of relying on the environment variable
* Also fixed a bug in test log output by prepending DIST_DIR to the
  output file path so that it can be grabbed by the server

Bug: 64687730
Test: run run_host_unit_tests script in both
        1) vanilla environment without envsetup and lunch
        2) fulled setup and lunched environemt
      to verify
        1) If nothing is setup, the script picks a default environment
	2) If envirnoment is already setup, use existing environment
	   instead of creating new one

Change-Id: Ife28db509fe9af55cef26c7b480196efae39d3aa