Add input content checking to some build scripts

Some build steps, particularly javac, have really loose input rules.
I.e. javac steps are re-built when any input jar changes. Often, this
leads to unnecessary rebuilds of all the following steps.

Other build tools (ninja, goma), will check the contents of the inputs
to a step, and if those inputs haven't changed that tool doesn't
actually re-run the command for creating the output.

This change brings that same benefit to some of the Android python
build scripts. Particularly those that will save a significant amount
of time by adding input content checks.

The checking checks both the input files and the command that will be
run. It compares this against a stored md5 digest. If it has not
changed, then the output does not need to be recreated (though it is
still touched to trigger following steps).

BUG=158821


Review URL: https://chromiumcodereview.appspot.com/13432002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@192265 0039d316-1c4b-4281-b951-d872f2087c98

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