Hidden API blacklisting killswitch.

Just support "*" for now, meaning disable all API blacklisting for all
apps.

Test: Manually verified by:
- installing test app that accesses hidden API
- manually blacklist the API
- $ adb shell settings put global hidden_api_blacklist_exemptions \\*

This is a cherry pick of change I9a41a104742c9aaaf3a753e7b0f3a1106e37d4d3
from internal master.

Merged-In: I9a41a104742c9aaaf3a753e7b0f3a1106e37d4d3
Change-Id: Ibb2a1d054c1b099fc6093e37f24b2ed421a2313c