Start using libmeminfo for all memory stats gathering.

This begins the change by gathering the system memory info
in android_os_Debug. The memory stats parsing code is planned
to get centralized in system/core/libmeminfo. So, we can also
add things like

 1. 'dumpsys meminfo -w' to get the working set.
 2. Use library to take a diff between 2 working sets of a process.
 3. Provide WSS / PSS monitoring via library as well.

Have all of memory info parsing code unified, tested and benchmarked
in one place so any changes can be tracked, reverted, modified easily.

Bug: 114325007
Bug: 111694435
Test: dumpsys meminfo (compare with /proc/meminfo for accuracy)

Change-Id: I6600bbce007de38dcba890905b466dfe2622aa2c
Signed-off-by: Sandeep Patil <sspatil@google.com>