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
diff --git a/dexpreopt/config.go b/dexpreopt/config.go
index 0c79ccc..2a929c5 100644
--- a/dexpreopt/config.go
+++ b/dexpreopt/config.go
@@ -16,7 +16,6 @@
 
 import (
 	"encoding/json"
-	"io/ioutil"
 	"strings"
 
 	"android/soong/android"
@@ -185,7 +184,7 @@
 // soongConfig argument. LoadGlobalConfig is used directly in Soong and in
 // dexpreopt_gen called from Make to read the $OUT/dexpreopt.config written by
 // Make.
-func LoadGlobalConfig(ctx android.PathContext, path string, soongConfig GlobalSoongConfig) (GlobalConfig, []byte, error) {
+func LoadGlobalConfig(ctx android.PathContext, data []byte, soongConfig GlobalSoongConfig) (GlobalConfig, error) {
 	type GlobalJSONConfig struct {
 		GlobalConfig
 
@@ -196,9 +195,9 @@
 	}
 
 	config := GlobalJSONConfig{}
-	data, err := loadConfig(ctx, path, &config)
+	err := json.Unmarshal(data, &config)
 	if err != nil {
-		return config.GlobalConfig, nil, err
+		return config.GlobalConfig, err
 	}
 
 	// Construct paths that require a PathContext.
@@ -209,13 +208,13 @@
 	// either CreateGlobalSoongConfig or LoadGlobalSoongConfig).
 	config.GlobalConfig.SoongConfig = soongConfig
 
-	return config.GlobalConfig, data, nil
+	return config.GlobalConfig, nil
 }
 
 // LoadModuleConfig reads a per-module dexpreopt.config file into a ModuleConfig struct.  It is not used in Soong, which
 // receives a ModuleConfig struct directly from java/dexpreopt.go.  It is used in dexpreopt_gen called from oMake to
 // read the module dexpreopt.config written by Make.
-func LoadModuleConfig(ctx android.PathContext, path string) (ModuleConfig, error) {
+func LoadModuleConfig(ctx android.PathContext, data []byte) (ModuleConfig, error) {
 	type ModuleJSONConfig struct {
 		ModuleConfig
 
@@ -233,7 +232,7 @@
 
 	config := ModuleJSONConfig{}
 
-	_, err := loadConfig(ctx, path, &config)
+	err := json.Unmarshal(data, &config)
 	if err != nil {
 		return config.ModuleConfig, err
 	}
@@ -289,10 +288,10 @@
 
 // LoadGlobalSoongConfig reads the dexpreopt_soong.config file into a
 // GlobalSoongConfig struct. It is only used in dexpreopt_gen.
-func LoadGlobalSoongConfig(ctx android.PathContext, path string) (GlobalSoongConfig, error) {
+func LoadGlobalSoongConfig(ctx android.PathContext, data []byte) (GlobalSoongConfig, error) {
 	var jc globalJsonSoongConfig
 
-	_, err := loadConfig(ctx, path, &jc)
+	err := json.Unmarshal(data, &jc)
 	if err != nil {
 		return GlobalSoongConfig{}, err
 	}
@@ -352,26 +351,6 @@
 	}, " "))
 }
 
-func loadConfig(ctx android.PathContext, path string, config interface{}) ([]byte, error) {
-	r, err := ctx.Fs().Open(path)
-	if err != nil {
-		return nil, err
-	}
-	defer r.Close()
-
-	data, err := ioutil.ReadAll(r)
-	if err != nil {
-		return nil, err
-	}
-
-	err = json.Unmarshal(data, config)
-	if err != nil {
-		return nil, err
-	}
-
-	return data, nil
-}
-
 func GlobalConfigForTests(ctx android.PathContext) GlobalConfig {
 	return GlobalConfig{
 		DisablePreopt:                      false,