libmeminfo: Add support for counting swap pages

Adds procmeminfo API to get the vector of swap_offsets
if SWAP is enabled on the device.

Bug: 114325007
Bug: 111694435
Test: libmeminfo_test 1
Change-Id: If0b52d042749a5bcb2c87aa2cb1595190d4769b1
Signed-off-by: Sandeep Patil <sspatil@google.com>