Revert "Revert "Remove legacy-test and legacy-android-test targets""
Fixes up the android.test.legacy target to stop using sources directly
now that the issue with android.test.runner-minus-junit not being able
to be built against the SDK has been resolved.
Also, fixes a couple of comments.
This reverts commit 7d7af10dc9.
Reason for revert: When this was originally applied it broke the
pi-dev-plus-aosp branch as it contained references to these targets.
Those references have now been removed.
Test: make checkbuild