android: Disable trying to read/write to the disk cache.

We need the disk cache enabled in Android to get EGL_ANDROID_blob_cache's
callbacks called, but we don't actually want to store anything on disk.
Fixes "Failed to create //.cache for shader cache (Read-only file
system)---disabling." spam on init.

Reviewed-by: Tapani Pälli <tapani.palli@intel.com>
Reviewed-by: Kristian H. Kristensen <hoegsberg@google.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6762>