improvement for emma filter option

the original implementation directly embeds the filter variable into command
line; this implies that filter variable cannot contain any whitespaces,
otherwise it'll be misintepreted as a new command line argument to emma. the
side effect is that user will be forced to cram all filter related java
class/package names into one line, which could be very long. with this
change, user can use line continuation ("\") or appending ("+=") to break
up long filter specs.

Change-Id: I2c130033cdb5120251d6f84c3dd906d140d52991