Since unit tests only run on eng builds, modify the proguard config to
only keep the methods/classes needed for unit tests in eng builds
The test proguard config probably is keeping too many methods. We should
also evaluate whether it is valid for unit test to use methods that are not
used in user builds
The resulting apk size is 8714188 bytes, from 9866129
According to the proguard output, the final number of program classes is
4409, from the previous count of 10932
Change-Id: Ie7065a454e1ff80a4b3f953684940d88b17f6db7
