Show reasons for regenerating the ninja file

Adding env SOONG_UI_NINJA_ARGS="-d explain" for elaborating the reasons
for generating the ninja file.

Bug: 271654778
Test: touch tools/asuite/atest/Android.bp; atest-dev hello_world_test
 --host
Change-Id: Ifa858b2d302bed642a5a789c50ab954c8e547e5a
1 file changed