Provide a complete definition of FileBlobCache

```
In file included from external/libcxx/include/algorithm:644:
external/libcxx/include/memory:2335:19: error: invalid application of 'sizeof' to an incomplete type 'android::FileBlobCache'
 2335 |     static_assert(sizeof(_Tp) > 0,
      |                   ^~~~~~~~~~~
external/libcxx/include/memory:2652:7: note: in instantiation of member function 'std::default_delete<android::FileBlobCache>::operator()' requested here
 2652 |       __ptr_.second()(__tmp);
      |       ^
external/libcxx/include/memory:2606:19: note: in instantiation of member function 'std::unique_ptr<android::FileBlobCache>::reset' requested here
 2606 |   ~unique_ptr() { reset(); }
      |                   ^
frameworks/base/libs/hwui/pipeline/skia/ShaderCache.h:40:7: note: in instantiation of member function 'std::unique_ptr<android::FileBlobCache>::~unique_ptr' requested here
   40 | class ShaderCache : public GrContextOptions::PersistentCache {
      |       ^
frameworks/base/libs/hwui/pipeline/skia/ShaderCache.h:35:7: note: forward declaration of 'android::FileBlobCache'
   35 | class FileBlobCache;
      |       ^
1 error generated.
```

Test: Build
Fixes: b/328705924
Change-Id: I6396f5fafef1941201c05d0b107f48e6d28a6631