Stop font loading with restricted context.

Resources.getFont may load other package's font. Stop loading font
resources unless the developer allowed to do so.

Bug: 64115349
Bug: 35763094
Bug: 62813533
Test: bit CtsTextTestCases:android.text.style.cts.TextAppearanceSpanTest
Change-Id: Ifa1b57a70650ba086b38407c0ed5b4048983e7e5