Build - 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: I1d219ed500eec3c6e789733f07269d32a76c9218
Signed-off-by: STELIX <ssspinni@gmail.com>