Add a return value to BlobCache::set

HWUI's shader cache uses a BlobCache for writing to persistent storage.
We would like to know whether the cache is large enough to hold the
necessary shaders, but BlobCache does not currently provide any way to
inspect it.

Add a new enum with possible outcomes from BlobCache::set and return it
from that method. This way a client can use this information, e.g. in
perfetto traces.

Bug: 231194869
Bug: 225211273
Test: atest libEGL_test (BlobCacheTest)
Change-Id: I9aade9da42cae83da053cc8d5e24999de1936de6