Report ION usage event when ION heap size is 0

In a situation when ION heap size is 0 dumpsys meminfo will not report
ION memory usage. Fix this by differentiating 0-size ION heap or pool
from the cases when /sys/kernel/ion/* do not exist or not readable.

Bug: 168942451
Test: emulate 0-size ION heap and run dumpsys meminfo
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Change-Id: I66624c9e3c73a59462cce8830a36323677db0029