Fix java version detection when _JAVA_OPTIONS is set.

_JAVA_OPTIONS is an environment variable that
can be used to affect the behavior of java and javac.
It is currently required to get Android to build on
some configurations, where the default Java heap size
is too small. Unfortunately, if _JAVA_OPTIONS is set,
both java and javac will output its value to the console
as the first line on every invocation, including trivial
ones like java -version. This will confuse main.mk’s
version detection, which only looks at the first line of
output. Tweak the version detection to run grep before
head, so that the _JAVA_OPTIONS line is filtered by the grep.

Change-Id: I69aee52b56d27711b7d3087ec6b3ebab07ffc3af