EGL: Close Multifile Blobcache files after mapping

When loading entries from disk, we don't need to keep the
files open after mapping their contents.

Per mmap documentation:

  After the mmap() call has returned, the file descriptor, fd, can
  be closed immediately without invalidating the mapping.

  https://man7.org/linux/man-pages/man2/mmap.2.html

This will prevent consuming excessive file descriptors, which
are a limited resource.

Added new test that ensures file descriptors do not remain open.

Test: libEGL_test, EGL_test, restricted_trace_perf.py
Bug: b/286809755
Change-Id: I6317fdbce340a8e7cbf3020ad41386cf9915dd2d