Create Make flags to set source tree as ReadOnly in soong builds

The following two Make vars control RO/RW access to the source tree
1. BUILD_BROKEN_SRC_DIR_IS_WRITABLE
2. BUILD_BROKEN_SRC_DIR_RW_ALLOWLIST

By default, (1) will be truthy.
- this ensures that this CL is a non breaking change across all products
- different products can opt in to set is as "false"

Bug: 174726238
Test: from build/soong dir, ran go test ./ui/build
Change-Id: I4d55ac74f02b2a73194d31506a9010162620b25a
diff --git a/ui/build/config.go b/ui/build/config.go
index 1d1f71f..f9f638c 100644
--- a/ui/build/config.go
+++ b/ui/build/config.go
@@ -57,6 +57,7 @@
 	katiSuffix      string
 	targetDevice    string
 	targetDeviceDir string
+	sandboxConfig   *SandboxConfig
 
 	// Autodetected
 	totalRAM uint64
@@ -120,7 +121,8 @@
 
 func NewConfig(ctx Context, args ...string) Config {
 	ret := &configImpl{
-		environ: OsEnvironment(),
+		environ:       OsEnvironment(),
+		sandboxConfig: &SandboxConfig{},
 	}
 
 	// Default matching ninja