Check extracted dex only once per usage

The check is unnecessary in MultiDex.install because it was already done
by MultiDexExtractor.load. The retry on bad extraction is also included
in MultiDexExtractor.load so it was redundant too.

Test: SupportMultidexHostTest (from tradefed)
Change-Id: I877a99db0e0c562ac47a7c5c87d7f3e1d70884e6