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android: Document more GN build rules + minor cleanups.

This CL tries to document more internal GN build rules,
as a preliminary for future patches that will perform
refactoring of some of them. In particular to simplify
the impressive java_library_impl() rule.

Also perform a little cleanup, since this led to
discover a few obsolete things:

- Remove the 'manifest_entries' variable since it is
  never set, and its entries passed as --manifest-entry=
  arguments to javac.py, which doesn't support it!

- Remove forwarding of the optional public_deps variable,
  in cases where it would never be used by the final
  rules that receive it (it was never being set by any
  upstream callers).

- Remove un-needed passing of processors_javac which
  seems obsolete.

- Move more variables in the (type == "android_apk")
  block of java_library_impl() to make it clearer that
  these are only used for android_apk() targets.

BUG=830641
R=agrieve@chromium.org

Change-Id: I489c1921d58bc9f649a5bc02cf630c36e607b20a
Reviewed-on: https://chromium-review.googlesource.com/1007061
Commit-Queue: David Turner <digit@chromium.org>
Reviewed-by: agrieve <agrieve@chromium.org>
Cr-Commit-Position: refs/heads/master@{#550182}