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

Fixes: 30b38df5 ("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
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Change-Id: I37ccdd4ecc97e9c7e10728834a56b422ac18c3c7