Export TARGET_JAVAC and HOST_JAVAC to make. This moves control
of the javac command line flags into soong.
Test: m -j checkbuild
Merged-In: Ifa3e35be0abd2778741680216427403ba58a0a3b
Change-Id: Ifa3e35be0abd2778741680216427403ba58a0a3b
(cherry picked from commit 2478d8b885)