Adding OEM Network Preferences for testing

Added two new per-app OEM network preferences to be used only with
testing.

Although there is a tradeoff adding these preferences to
non-test code, the benefit is that it will allow testing of the OEM
network preferences app via CTS as using the shell with elevated
permissions is not an option due to the OEM network preferences API
potentially being abused. On consideration, a small amount of test code
was deemed an acceptable tradeoff if it enabled CTS testing.

Bug: 176496580
Bug: 176494815
Test: atest FrameworksNetTests
atest FrameworksNetIntegrationTests
atest CtsNetTestCasesLatestSdk

Change-Id: Ie896a7b40945a46a352f37d2c00671da8a8f4489