Add --replace-version flag to aapt.

Motivation: we'd like to programmatically specify the version
name/code (i.e. to include the build number from the build server).
However, this means that we cannot specify version info in the
AndroidManifest.xml file, as this takes precedence. Not doing so makes
IDE use more difficult, as the IDE gets version code 0 and won't
install over an existing non-IDE version unless you first force a
downgrade to an IDE build from the command line.

This flag allows us to specify a very high version code in the
AndroidManifest.xml file, making IDE builds take precedence, while
still allowing us to override this info when performing command-line
(official) builds.

Change-Id: I5d01048698af5c26bdf19066c6cd4eca1115112a