Fix bad BUILD file and failed test for AaptGen tests

This is independent of any other of my reviews currently up.

This test was broken, had been for a little while. A change broke
the test but it had an inconsistent BUILD file so that
test was never getting called by the CI.

I have a somwhat large refactor of the entire Aapt pipeline
coming up with the AaptBuild task, which I am writing tests
for now. But I wanted to get this in soon, because the this test
will fail if the user calls something like
'./pants goal test tests::', etc.

Testing Done:
Verified that all android tests are being called by CI.
CI passes.
Travis passes.

Reviewed at https://rbcommons.com/s/twitter/r/858/
Simplify MultipleRootedProducts._get_products_for_root by using setdefault

Testing Done:
./pants goal test tests/python/pants_test:all

Reviewed at https://rbcommons.com/s/twitter/r/851/
docs: Remove more references to /BUILD:target notation

Found with git grep 'BUILD:'. Other uses still exist, but they seem to be in
example output.

Reviewed at https://rbcommons.com/s/twitter/r/855/
Add DxCompile task to android backend

This task gathers the class files of a target and compiles them into a classes.dex.
Dex file is a Dalvik exeecutable file. Once the classes have been dexed, then the app
is ready to be packaged into an .apk.

The classes.dex that is created is output to
pants.d/dex/dex/target.id/classes.dex. The target.id is
simply the package name. This is done b/c every Android target that
is dexed will output a file named "classes.dex". Putting them in
dirs that are named after the target.id means that they will
not clobber each other unless the package names correspond
(which would cause other problems as well).

CLI options allow configuring of VM options and build-tools versions.

This is just the task itself, I will write the test class tomorrow. I wanted to give people
a chance to comment while I worked on them, in case large changes are needed.
If you prefer to wait for the test class, that should be up tomorrow.

Testing Done:
CI.sh passes at 67089e7
Full integration tests pass on local machine.

Travis now passes [0].

[0] https://travis-ci.org/pantsbuild/pants/jobs/31952336

Reviewed at https://rbcommons.com/s/twitter/r/840/
Add a try/raise block to catch exceptions from calling os module on None

If someone defines an Android target without specifying a 'manifest'
field in the BUILD file, they only got a cryptic error. The exception was
never called because the error came earlier, when calling os.path on a None
object. This ensures that the user sees the error message we mean them to.

I have added testing for Android targets to my TODO list.

This patch is small and shouldn't be dependent on any other open reviews.

Testing Done:
CI passes.
Did some fuzz testing manually.
Travis passes. [0]

[0] https://travis-ci.org/pantsbuild/pants/builds/32069220

Reviewed at https://rbcommons.com/s/twitter/r/860/
Remove unused invalidate_for method and unused extra_data variable.

As discussed in https://rbcommons.com/s/twitter/r/808/ we are removing the unused extra_data variable and invalidate_for.

Testing Done:
Added an integration test to ensure java compile with 6 and then 7 produces two different artifacts.

Reviewed at https://rbcommons.com/s/twitter/r/849/
Prepare for 0.0.23 release.

Testing Done:
None

Bugs closed: 469

Reviewed at https://rbcommons.com/s/twitter/r/862/
Merge branch 'master' into aapt-refactor-tests

Conflicts:
	src/python/pants/backend/android/register.py
	src/python/pants/backend/android/tasks/BUILD
	src/python/pants/backend/android/tasks/dx_compile.py
	tests/python/pants_test/android/BUILD
	tests/python/pants_test/android/tasks/BUILD
	tests/python/pants_test/android/tasks/test_aapt_gen.py
	tests/python/pants_test/android/tasks/test_dx_compile_integration.py