Add prebuilt_etc_xml

prebuilt_etc_xml installs an xml file to <partition>/etc[/<subdir>] and
validates the xml file against the given DTD file before installing it.

This change also includes some fixes for prebuilt_etc which is the super
module of prebuilt_etc_xml:

1) The module is changed to arch-specific module as the prebuilts are only
for devices (installed under the etc dir), but not for hosts.

2) Dependency to android.Prebuilt is removed because android.Prebuilt is
intended to be used for the case when a module can exist as prebuilts,
source or both. These prebuilt_etc_* modules are prebuilt only.

3) srcs property which accepts a list of source files is changed to src
that only accepts single source file, which makes more sense for
prebuilts.

Bug: 65686190
Test: m -j (xml_test.go)
Change-Id: I40484f3f6615b99f6b8d43176db0c40c5bfd838e