Add --skip-make to replace Soong's blueprint wrapper with soong_ui

This way we only have one way to start a build, which always has logging
/ tracing / etc, even if we don't need Kati.

There's two ways to use this:

As a direct replacement for mkdir out; cd out; ../bootstrap.bash;
./soong -- as long as --skip-make is always passed, we'll never run
Kati, and Soong will run outside of it's "make" mode. This preserves
most of the speed, and allows full user control over the Soong
configuration.

A (experimental, dangerous) way to temporarily bypass the product
variable and kati steps of a build. As long as a user is sure that
nothing has changed from the last build, and they know exactly which
Ninja targets they want to build (which may not be the same as the
arguments normally passed to 'm'), this can lead to shorter build
startup times.

Test: rm -rf out; m --skip-make libc
Test: rm -rf out; m libc; m --skip-make libc
Test: rm -rf out; mkdir out; cd out; ../bootstrap.bash; ./soong libc
Test: build/soong/scripts/build-ndk-prebuilts.sh
Change-Id: Ic0f91167b5779dba3f248a379fbaac67a75a946e

scripts/build-ndk-prebuilts.sh

@@ -54,8 +54,7 @@ cat > ${SOONG_OUT}/soong.variables << EOF
     "Safestack": false
 }
 EOF
-BUILDDIR=${SOONG_OUT} ./bootstrap.bash
-${SOONG_OUT}/soong ${SOONG_OUT}/ndk.timestamp
+m --skip-make ${SOONG_OUT}/ndk.timestamp
 
 if [ -n "${DIST_DIR}" ]; then
     mkdir -p ${DIST_DIR} || true