adbd: actually dynamically link against libadbd_auth.

libadbd_auth was accidentally linked as a static library, which defeats
the entire purpose.

Test: treehugger
Test: readelf -d $ANDROID_PRODUCT_OUT/apex/com.android.adbd/bin/adbd
Change-Id: Id5b66745946a8b92e368ee5f3965275ff03e056d