Make /proc/pressure/memory rw for all

The file is safe to write to, as it allows monitoring system pressure.
More info: https://docs.kernel.org/accounting/psi.html

Bug: 419532852
Test: boot device, check VM running, pressure system, look at logcat.
Flag: EXEMPT ...
Change-Id: I3dd781252371ec7f8925ff53450ad6417bd713f4