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

ui/build/Android.bp

@@ -60,6 +60,7 @@ bootstrap_go_package {
         "path.go",
         "proc_sync.go",
         "rbe.go",
+        "sandbox_config.go",
         "signal.go",
         "soong.go",
         "test_build.go",
@@ -86,5 +87,8 @@ bootstrap_go_package {
             "config_linux.go",
             "sandbox_linux.go",
         ],
+        testSrcs: [
+            "sandbox_linux_test.go",
+        ],
     },
 }