Add logging for FLAG_SLIPPERY usage

We are interested to learn about the usage of FLAG_SLIPPERY. Do any apps
use it? Or is it only used by system components?

If apps aren't using it, let's lock it down.

Bug: 157929241
Test: adb shell dumpsys platform_compat
Change-Id: I73130ebe86ea66906cd8f281785dc6aced1ce03f