dalvik/libdex: Don't reduce optimization level to -O2

Don't reduce optimization level to -O2: It's sufficient to disable
function inlining

Rebased from: http://review.cyanogenmod.org/#/c/26538
Change-Id: Ie6d9978cf0bf0e49cfa296c09c4eba913f22c303
Author: Bernhard Rosenkraenzer <Bernhard.Rosenkranzer@linaro.org>
Signed-off-by: Bernhard Rosenkraenzer <Bernhard.Rosenkranzer@linaro.org>