Add a helpful error message if GetUnreachableMemory fails

The primary reason for GetUnreachableMemory failing is selinux
blocking system apps from ptracing themselves.  Add a helpful
error message suggesting setenforce 0.

Bug: 27945735
Test: dumpsys meminfo --unreachable com.android.systemui
Change-Id: Ibfa9507860be36dffd514f82027f7a2f8a72ac48
Merged-In: Ibfa9507860be36dffd514f82027f7a2f8a72ac48
(cherry picked from commit 72d38814)