Add comments on UnsupportedAppUsage of Typeface.sDefault

Added comments and trackingBug for UnsupportedAppUsage annotation of
Typeface.sDefault.
Developers can already use theme to customize font for their app.
But it still has a huge amount of usage.
For that reason we can't put it into black list now, and further
investigate is required for future action.

Bug: 123769446
Test: atest TypefaceTest
Change-Id: I238c4dfbbf4a4df6a0c477d36560a28cf4dc081b