Fix concurrent access to Flag Cache.

A rare NPE sometimes occurs in FeatureFlagsClassicDebug. My best
hypothesis is that it occurs when one thread was attempting to read
the internal flag cache while another was adding an item to the cache.

The new code in this cl replaces the TreeMap with a ConcurrentHashMap.
In addition, it first tries to read the value out of the cache instead
of checking for a key's existence and then branching accordingly.

If the read fails, it determins what the value should be and adds it
to the cache, with no additionaly locks necessary beyond the internals
of `ConcurrentHashMap#put`. This means it is theoretically possible
for a flag to be `#put` twice if it is read from two different threads
simultaneously, but the odds of this are vanishingly small in addition
to being completely harmless.

Test: atest && manually built and run SystemUI
Fixes: 299029193
Change-Id: Iaaa7431dfdc454f6cacdb8561ae07d5fd9e44983