libcutils: test: increase use of ashmem_valid(fd)

There may be evidence of ashmem_valid(fd) reporting that the file
descriptor is an ashmem node.  Increase testing of ashmem_valid(fd),
reporting that the node _is_ ashmem, to inspire confidence in the
positive result.  Scan all file descriptors in the system, and for
those that pass ashmem_valid, get a non-zero size reference back.

Some clang-format-isms applied.

Test: libcutils-test --gtest_filter=AshmemTest.*
Bug: 72021458
Change-Id: I77d746b57a89a6afa1b829dddfdc4dd319f6b684