Add API for proxy configuration over VPN.

Test: runtest -x
frameworks/base/tests/net/java/com/android/server/ConnectivityServiceTest.java
&& atest HostsideVpnTests
Bug: 76001058
Change-Id: Id4dde4a4103fd93bfbbacc52d0e5ade56ae67a6a
Merged-In: Id4dde4a4103fd93bfbbacc52d0e5ade56ae67a6a