Use -processorpath for annotation processors

ErrorProne needs to be added to -processorpath, but once anything is
passed via -processorpath javac stops looking in the classpath for
annotation processors.  Move annotation processors to -processorpath
in preparation.

This reapplies Ife06b3c7432ab971f5605dcc18a558bcbd99a675 with fixes
to processorpath in errorprone rules.

Test: m java
Change-Id: I188136771386a2183fb31f4b70b50c95ac419ff8