Use new soong support to install privapp xml

This allows better overriding of it in the variants of this apex
that have a different package name.

Bug: 242509786
Test: build google tethering, diff before & after
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:91442c93fb8d7f67bf90faab0552428ff07f0452)
Merged-In: I84c53458686f70408ec759df854f5fcdee9cdaed
Change-Id: I84c53458686f70408ec759df854f5fcdee9cdaed