ANDROID: staging: android: ion: Fix build when CONFIG_ION_SYSTEM_HEAP=n

Fixes: 5e3a3fe770e6 ("ANDROID: staging: android: ion: Expose total heap and pool sizes via sysfs")
When CONFIG_ION_SYSTEM_HEAP not set ion_page_pool_nr_pages symbol will not
be compiled. Fix this by providing an implementation for this configuration.

Bug: 138148041
Test: build with CONFIG_ION_SYSTEM_HEAP=n
Change-Id: I77b87e8cd6789a06336c724c206acc832b302d08
Signed-off-by: Suren Baghdasaryan <surenb@google.com>