Ensure that AT_SECURE=1 is set when installd executes dex2oat.

LD_PRELOAD is no longer set by init, and installd couldn't see
LD_PRELOAD anyway due to https://android-review.googlesource.com/129971 .
Drop it.

Continuation of commit b00a0379

Change-Id: Icaf08768b3354c6a99dd0f77fef547a706cc96e9