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