Fix snapshot symlinks

LOCAL_MODULE_SYMLINKS was enough to install symlinks for snapshot
binaries. However, Soong now requires LOCAL_SOONG_INSTALLED_SYMLINKS,
which results in broken symlink support in snapshots.

This fixes symlink problems, and then amends the Soong snapshot test.

Bug: 220639435
Bug: 220907049
Test: manually add symlinks and try building
Test: Soong test
Change-Id: Ia75ddf3aa642558e396f1d80736310c3c8d02ad1