simpleperf: fix report_sample_test.

When running cmd_report_sample.* tests, it may call
ReportSampleCmd()->Run() more than once. If we shutdown
protobuf library in one call, using protobuf in the
following calls will encounter failures.

Bug: http://b/37996565
Test: run CtsSimpleperfTestCases.
Change-Id: I5ca217406e786645d45fda518055c4a63ab3db3c
1 file changed