SDK Manager: support extras/vendor/path

SDK Manager: support extras/vendor/path

This change makes the SDK manager install new extra
packages into SDK/extras/vendor/path, creating the
folders as needed.

Existing extra packages are not moved when they are updated,
provided that they keep the same 'vendor' and 'path' attributes
in their XML definition.

There is also some support for the form SDK/vendor-path/ that
was introduced by Tools r8 and r9.

SDK Bug: 14493

Change-Id: I28b301a768ea2c8c03573f865520b1b3e85f3868