Sandbox soong_build by changing to root directory

This relands I12a0f907753fefd1997ab8b4ea2ac331234093cf along with
a fix to blueprint for absolute paths.

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.

Change-Id: I24ac485677aa102eec1a2521d16820da6ee1ae77
Fixes: 146437378
Test: m checkbuild
Test: m OUT_DIR=/tmp/out nothing

android/module.go

@@ -16,13 +16,13 @@ package android
 
 import (
 	"fmt"
+	"os"
 	"path"
 	"path/filepath"
 	"strings"
 	"text/scanner"
 
 	"github.com/google/blueprint"
-	"github.com/google/blueprint/pathtools"
 	"github.com/google/blueprint/proptools"
 )
 
@@ -91,7 +91,8 @@ type EarlyModuleContext interface {
 
 	Glob(globPattern string, excludes []string) Paths
 	GlobFiles(globPattern string, excludes []string) Paths
-	Fs() pathtools.FileSystem
+	IsSymlink(path Path) bool
+	Readlink(path Path) string
 }
 
 // BaseModuleContext is the same as blueprint.BaseModuleContext except that Config() returns
@@ -1172,6 +1173,22 @@ func (e *earlyModuleContext) GlobFiles(globPattern string, excludes []string) Pa
 	return pathsForModuleSrcFromFullPath(e, ret, false)
 }
 
+func (b *earlyModuleContext) IsSymlink(path Path) bool {
+	fileInfo, err := b.config.fs.Lstat(path.String())
+	if err != nil {
+		b.ModuleErrorf("os.Lstat(%q) failed: %s", path.String(), err)
+	}
+	return fileInfo.Mode()&os.ModeSymlink == os.ModeSymlink
+}
+
+func (b *earlyModuleContext) Readlink(path Path) string {
+	dest, err := b.config.fs.Readlink(path.String())
+	if err != nil {
+		b.ModuleErrorf("os.Readlink(%q) failed: %s", path.String(), err)
+	}
+	return dest
+}
+
 func (e *earlyModuleContext) Module() Module {
 	module, _ := e.EarlyModuleContext.Module().(Module)
 	return module