Embed minibp into soong_ui.

This requires linking Blueprint into soong_ui. It lets us avoid the
complicated dance of Ninja files and shell scripts: now the information
as to how soong_build is built is passed directly to Blueprint using a
struct that contains all the information the command line arguments used
to contain.

The ability to run Blueprint from the command line is kept (for now).

Some variables in bootstrap/command.go needed public accessor functions
because soong_build reads them. This will be disentangled by moving the
flag parsing to soong_build.

The presence of the flag definitions in Blueprint means that soong_ui
now also accepts them. This is not a problem in practice because they
are ignored and because soong_ui itself is hidden behind a few layers of
shell scripts.

Test: Presubmits + the new bootstrap_test.sh .
Change-Id: I6dca478f356f56a8aee1e457d71439272351390b

ui/build/Android.bp

@@ -32,6 +32,8 @@ bootstrap_go_package {
     name: "soong-ui-build",
     pkgPath: "android/soong/ui/build",
     deps: [
+        "blueprint",
+        "blueprint-bootstrap",
         "soong-ui-build-paths",
         "soong-ui-logger",
         "soong-ui-metrics",