releasetools: don't create prebuilt_dir path if it exits

This path may already exists from previous build tasks. Attempting
to create it again will cause an error.

Change-Id: I4baad8bc846470b99f6ce9207c0682401406decc
(cherry picked from commit adccf33e0fba5b4fd6ed487894e5fee77274a4ef)
build: Degreenify java

Change-Id: Iac348407997f7e52844af0494ee59762df0819d7
(cherry picked from commit e50b108fdc59327bb29d27ebd82cfd70ddf01628)