Fix code coverage for apps

When building apps with EMMA_INSTRUMENT=true, we must ensure that
jacoco is at least on the classpath (or even embedded for unbundled
branches) and that proguard will ignore jacoco classes. Otherwise
the Jack compilation would fail.

For unbundled build, we include the proguard configuration file
build/core/proguard.jacoco.flags to ignore jacoco classes.

For full build, we ensure jacoco is on the classpath during the
jack compilation.

Bug: 27607712
Change-Id: I0ad37d74e63028a8144a125f052dc1c5fa10fc66