Adjust procfs heap buffer growth limit to 64 MiB

The current 1 GiB limit is excessive, and would result in extreme
amounts of copying for very large files. Files read by
android_os_Process_readProcFile should be well under this new limit,
and most should only use the stack buffer anyways.

Bug: 351917521
Test: cuttlefish boot
Change-Id: I37da934b5e66d9b56da928dabd7e115e2ed6dca7