Stubs libs are dist artifacts

stubs libs from a java_sdk_library are automatically registered as dist
artifacts for sdk and win_sdk targets.

They are installed under $(DIST_DIR)/apistubs/<apiscope>/<name>.jar
where <apiscope> can be public, system and test.

Bug: 605033
Test: m -j
Test: choosecombo 1 sdk_phone_armv7 userdebug; m -j sdk dist
the stubs libs are found under out/dist/apistubs

Change-Id: I3487634f1ec5bb3b5b60d636b64461d391add35a