simpleperf: fix printing kernel address warning.

1. In cmd_record.cpp, don't read kernel addresses when kernel samples
are not recorded.
2. In kallsyms.cpp, print kernel address warning only once.
3. In kallsyms.cpp, use property only when running as root.

Bug: none
Test: run simpleperf_unit_test
Change-Id: I06de729ff3f26f7cd2115593182d6cb9cdc7f525
7 files changed