Make /proc/sys/kernel/perf_event_max_sample_rate accessible to untrusted_app.

perf_event_max_sample_rate is needed to be read for native profiling,
otherwise CTS test can fail on devices with kernel >= 4.4. Before this CL,
the file is not readable from untrusted_app domain. This CL makes it readable
from both shell domain and untrusted_app domain.

Bug: http://b/35554543
Test: build and test on marlin.
Change-Id: Id118e06e3c800b70a749ab112e07a4ec24bb5975