Workaround build failure in coverage build

The jacocoagent is special as while it is depended on by prebuilts it
is not actually a prebuilt.

Bug: 177892522
Bug: 186118711
Test: m EMMA_INSTRUMENT=true EMMA_INSTRUMENT_FRAMEWORK=true nothing
      Previous command fails without this change and works with it.
Change-Id: I8fba922fc40919936d74a6f95af745e37f1ca61f