Enable `is_always_lock_free` assertion on RISC-V with Zacas.

The `__riscv_zacas` extension provides the necessary atomic operations for `CacheEntry` to be lock-free, so the assertion is no longer relaxed on RISC-V platforms that support it.

Flag: android.os.parcel_string_cache_enabled
Bug: 442920832

Change-Id: Id8cfd8dbc9dd764878e9140ff3b3b361a4b68be4