Enable multi-dex for dx

Add LOCAL_DX_FLAGS := --multi-dex where it is enabled for jack.
Also stop enabling multi-dex when coverge is enabled, since
multi-dex is now enalbed unconditionally.

Test: builds
Change-Id: I67fedcdd5bb5d7091468bddcb0c22bab720a51d2
Merged-In: I67fedcdd5bb5d7091468bddcb0c22bab720a51d2
(cherry picked from commit 17ffa242)