Do not set --lto-O0 for optimize_for_size targets

--lto-O0 produces smaller binaries compared to --lto-O2 under -O2.
However under -Oz (the default for optimize_for_size), --lto-O2 actually
produces smaller binaries.

                     --lto-O0       --lto-O2
StatsD               2796856        2673728
libLLVM_android.so   15552712       14595544

Test: m
Change-Id: I33151cfd6abc273c4836418a6d8e0b935db02af4