Rework the generation of host java libraries.

We currently have two types of host libraries, those
meant for the host VM (Sun, OpenJDK etc.) and those meant
for a host dalvik build. The former need to be compiled
against the host standard libraries and the latter need
to be compiled against libcore. This change introduces
two new build rules to complement the existing the existing
host rules.

BUILD_HOST_DALVIK_JAVA_LIBRARY : Build a java library for
a host build of dalvik. Bootclasspath will be set to a host
build of libcore.
BUILD_HOST_DALVIK_STATIC_JAVA_LIBRARY : Build a static java
library for a host build of dalvik. Bootclasspath will be set
to a host build of libcore.

This change also removes support for the LOCAL_BUILD_HOST_DEX
flag, which is now unnecessary.

bug: 8992787

Change-Id: I3569fff8eaa4d26d55fcc317bd98471f55d74c14