Sandbox soong_build by changing to root directory

Store the current working directory and then change to the root
directory so that all file accesses must go through helpers in
the android package that properly track dependencies.

Fixes: 146437378
Test: m checkbuild
Change-Id: I12a0f907753fefd1997ab8b4ea2ac331234093cf

android/register.go

@@ -84,7 +84,9 @@ type Context struct {
 }
 
 func NewContext() *Context {
-	return &Context{blueprint.NewContext()}
+	ctx := &Context{blueprint.NewContext()}
+	ctx.SetSrcDir(absSrcDir)
+	return ctx
 }
 
 func (ctx *Context) Register() {