Remove hidden API usage of Proxy.validate

Proxy is planned to move to the connectivity module, so framework
classes like DevicePolicyManager cannot depend on its private symbols.

Replace usage of ProxyInfo.validate by the equivalent isValid method on
ProxyInfo.

Bug: 174436414
Test: atest DevicePolicyManagerTest#testGetProxyParameters
Change-Id: I6da49a34c9893d07ef7cbee6a7386c9258839467