dx: De-dupe prototype reference classes

CstMethodType and CstProtoType covered the same purpose. Having two
classes for prototype references broke in the included test case.

Bug: b/63112734
Test: dalvik/dx/tests/run-test 139-lambda-metafactory
Change-Id: I240e410c50dda56baa88a27f6cba679bd8da22c2
8 files changed