Add linter for FlagManager flags

Add a presubmit linter to ensure all flags defined in FlagManager.cpp
are also dumped for debugging purposes.

The linter is a Python script that checks for missing flag dumps.
The TEST_MAPPING file is updated to run this script as a presubmit test.

Change-Id: Icb46839b31e015bb7d24025d075b101bd8ebd13f
Bug: N/A
Test: presubmit
Flag: EXEMPT test only