Don't install boot.art when WITH_DEXPREOPT is false

Don't create the install rules for boot.art when WITH_DEXPREOPT
is not set to true, which will ensure there is no dependency on
boot.art in a normal build, which matches the behavior before
I25163e91886cea6941afa25cdb529ed053278dcb.

This should fix the Mac build, which never sets WITH_DEXPREOPT
and does not support running dex2oat.

Bug: 119412419
Bug: 121130576
Bug: 121183382
Test: lunch aosp_sailfish-eng && m WITH_DEXPREOPT=false
      files does not depend on out/target/product/sailfish/dex_bootjars/system/framework/arm/boot.art
Change-Id: I39e96b1f35f6808d3007cf1785ec3db89e0decb6