Add a shared library for setting flags in SysUI.

This adds the FlagManager class which currently has 3 methods:
- #getFlagsFuture
- #setFlagValue
- #eraseFlag

The first makes an async call to SystemUI to retrieve the flags
it knows about. It returns a collection of Flag objects from which
one can get the id, type, and default values.

The other two are fairly self-explanatory, sending intents to
SystemUI telling to set or erase flags. Any app calling these
methods must have the FLAGS_PERMISSION defined in the FlagManager
library.

Only boolean flags are supported at the moment.

Bug: 203548827
Test: manual
Change-Id: Id15ff481edcb83c67ce4cdad6cdda27d28a40652