Fix Android.mk files for plugins on darwin.

Darwin uses a few different flags for this than Linux, and we missed this
when we updated the test builds recently.

Change-Id: I8a0e8bc18ef698a0ed933f96b2bce0653f4307e5
(cherry picked from commit 651de8d5ac0b28b10ad0259efc75cf6152dc13dc)
4 files changed