Allow custom compiler filters for system server jars

Passing down a compiler filter in LOCAL_DEX_PREOPT_FLAGS or
PRODUCT_DEX_PREOPT_DEFAULT_FLAGS will no longer always get
overridden to speed.

Bug: 62356545
Test: make and flash

Change-Id: I8b777c8881cba6b11cda56d27f0a714c92d70d94