releasetools: ota_from_target_files: add FullOTA_PostValidate

Change-Id: I152412049f90fd546d4516cc064238c3192be553
build: Ensure /system unmounted at install start

Account for the possibility that a user has mounted system manually
before installing the FullOTA package.

Change-Id: I97d0540676a7ce98762545e76ecb1a7f6335d204