simpleperf: fix reading filev2 section.

Without clearance, the dex_file_offsets of a dex file may be
wrongly inherted by the following elf files. So use
FileFeature::Clear() to clear state before reading FileFeature.

Bug: 192556680
Test: run simpleperf_unit_test
Change-Id: I735959360bca7b1a5cbf8dfb405688032976c34d
3 files changed