Label /proc/sys/kernel/pid_max as proc_pid_max.

And give shell domain read access to /proc/sys/kernel/pic_max.

Bug: 69569397
Test: adb shell /data/nativetest/bionic-unit-tests/bionic-unit-tests
--gtest_filter=pthread.pthread_mutex_owner_tid_limit
Change-Id: Ib56c18ed553ad2c2113e6913788a4c00965483cc