Add test for setHttpProxyConfiguration

The test verifies that system properties are set as expected by the
method.

Bug: 174436414
Test: atest ProxyTest
Original-Change: https://android-review.googlesource.com/1564956
Merged-In: Id15909c5e52f1042d7210cf7e416874bb01baee7
Change-Id: Id15909c5e52f1042d7210cf7e416874bb01baee7
2 files changed