Move clean/clobber to soong_ui

So that we don't have to load up all the makefile state just to remove
the output directory.

Starting from a completely empty out directory:
 kati:    16s
 soong_ui: 2.0s

From a minimal out directory (m -j blueprint_tools):
 kati:     3.8s
 soong_ui: 0.4s

Test: m -j clean
Test: m -j clobber
Change-Id: Iec1ce032c7cc7ed102430080e857fc421b66309c