Try to mount and compile system_ext apks.

On newer devices the /system_ext partition often contains apks that
should be compiled by otapreopt_chroot during OTA. This change makes
sure otapreopt_chroot is able to mount that directory in order to
access and compile the apks.

Test: forrest test -l remote --test_bench_config asit/redfin ab::<PBuild>:redfin-userdebug:git_master asit/dexoptota/self_full
Bug: 181601383
Bug: 181182967
Change-Id: Ifdf60f98c2afa373ab0beedd84ffb15cf89aa475