ANDROID: mm: vmscan: support equal reclaim for anon and file pages

When performing memory reclaim, support treating anonymous and
file backed pages equally. Swapping anonymous pages out to memory
can be efficient enough to justify treating anonymous and file backed
pages equally.
Create a vendor hook inside of get_scan_count so that equal reclaim of
anon and file pages can be enabled inside of the trace hook based on
certain conditions.

Bug: 175415908
Change-Id: Idac2f1468371549d20dd3e5652c7382dc3d7d9cf
Signed-off-by: Sudarshan Rajagopalan <sudaraja@codeaurora.org>