Add test for proxy spec building

In preparation for a refactor of the logic, add a test to verify proxy
spec building and validation in DevicePolicyManager#setGlobalProxy.

Bug: 174436414
Test: atest DevicePolicyManagerTest#testGetProxyParameters
Merged-In: Ic1c8bf6e8f5e79a7a1736af6cc1d50a7969fb9ad

Change-Id: Ic1c8bf6e8f5e79a7a1736af6cc1d50a7969fb9ad