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 bt, diff before & after
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:ebd1acbcf15286030d939908f6e4ebfaeb56b68c)
Merged-In: I54dd8373a74873276198c007a026ffb2ab8a8c12
Change-Id: I54dd8373a74873276198c007a026ffb2ab8a8c12