EGL BlobCache: Trim immediately on multifile overflow

While adding a fuzzer for this feature, discovered that
cache trimming wasn't happening right when the overflow
happened, but on the next iteration.

This CL updates the code to make it trim right away, since
we've already determined it needs to happen.

Also add a test to ensure trimming happens right away.

Test: /data/nativetest64/EGL_test/EGL_test
Bug: b/266725576
Change-Id: Ica1e9ca688268e7e746750c27acdfced04e74b06