ANDROID: mm: add config for default readahead size

Change the VM_MAX_READAHEAD value from the default 128KB to a
configurable value. This will allow the readahead window to grow to a
maximum size bigger than 128KB, which greatly benefits to sequential
read throughput and thus boot performance.

Bug: 62413151
Test: boot walleye 100ms faster
Change-Id: Iad448cf1198056de46654dcb409466802b3b908d
Signed-off-by: Wei Wang <wvw@google.com>