Add ability to set/erase flags via intents.

A flag can be set with a command like the following:
adb shell am broadcast -a com.android.systemui.action.SET_FLAG \
--ei id 1 --ez value 1

A flag can be "erased" with the same command, but leaving the value
unset.

Bug: 202860494
Test: manual
Change-Id: If1097e230692df49a6de7cbbb71ed37b0499c663