This commit is us getting out of our own way. We were specifying
-classpath twice, once in $(JAVAC) and once in java-build.mk. Only
the latter of these is active. This a problem for ANDROID_EXTRA_JARS
-- those JARs should be on the classpath and input to $(DX) -- and
JARs that should be on the classpath but *not* input to $(DX). This
commit removes the global flags to $(JAVAC) and adds
JAVA_{BOOT}CLASSPATH_JARS. This required some hijinkery moving
wildcards to moz.build files, but everything seems to work.
As well as clarifying some parts of the build, part 2 uses this work
to modify the classpath.
