Open source permissions

Summary:
This moves into open source the PermissionsModule and the activity and listener interfaces
necessary to make permissions work.
It also moves the PermissionsExample into the UIExplorer. In order to make this
work, the device has to be Android M and above, and the target sdk of the app has to be 23+, so I changed the uiexplorer manifest to
target that API. This has the unfortunate consequence that people testing on
devices with API 23+ will have to enable the `draw over other apps` setting that
react needs for RedBoxing. The app will automatically send the user to that screen,
so enabling the setting and resuming the app should be trivial.
For testing, try requesting permission for a permission that is currently
revoked. If a permission is granted, it can be revoked via adb (`adb shell pm
revoke com.your.app android.permission.PERMISSION_NAME`), and then requested.

Reviewed By: bestander

Differential Revision: D3431324

fbshipit-source-id: 8cbaea676d2b5727cb5191cdb77a02e213bf9ba3