simpleperf: avoid warning for missing dex files.

This avoids warnings like
"python3 W] Failed to read symbols from dex_file /system/framework.jar",
which are usually not useful, and may be confusing.

Bug: none
Test: run simpleperf manually
Test: run simpleperf_unit_test
Change-Id: Id64cb035ec9e00ffbe3bddb540a5ca20d96eeb9f
1 file changed