fs_mgr: allow for zramsize to be specified as percentage of total memory

If the zramsize entry has a % size at the end, as in the following example:

	/dev/block/zram0 none swap defaults zramsize=25%

then we will set the value as that percentage of total RAM, as read by scanning
/proc/meminfo.

b/20760266 Seed common build allocated too much for zRAM-backed swap

Change-Id: I17c91d311ba99ae7adae112bfe1b38542ea69b80
Signed-off-by: Iliyan Malchev <malchev@google.com>