Use EMMA_INSTRUMENT_FRAMEWORK for apex framework libs.

Static coverage builds that trigger a dexpreopt target for a bootclasspath
jar can fail since coverage for jars in apexes are turned on by default
which requires jacocoagent to be present on the bootclasspath.

Fix this by using EMMA_INSTRUMENT_FRAMEWORK to conditionally instrument
framework libs in apexes.

Bug: 157737183
Test: EMMA_INSTRUMENT=true EMMA_INSTRUMENT_STATIC=true m -j droid
Change-Id: I2c323553e08741bc46b196bc3bb860614bc3f85b