Allow override of DEX_PREOPT_DEFAULT

Allow device-specific override of DEX_PREOPT_DEFAULT value
rather than dependent on the number of runtimes supported
by the system.  This allows force generation of
non-stripped packages even when dex pre-opt is enabled
with only 1 runtime.

Change-Id: I57573ce17dc46b7eccf1a3619b073ead1b77c134