Make ReadDmabufHeapPoolsSizeKb/HeapTotalExportedKb fall back to ION

This will make it easier for new clients to use the API since they do
not have to be aware of whether the device supports ION on DMA-BUF
heaps.

Bug: 167709539
Test: manual
Change-Id: I692fb4b14d6c6bfe2707929a166bf77dc63e10dc
1 file changed