Re-work flatbuffer build and bundler

-Convert from .h to .cc for compilation unit
-Add libs to wrap generated files
-Add privacy to various targets
-Remove unneeded deps

Bug: 202841894
Tag: #refactor
Test: gd/cert/run
BYPASS_LONG_LINES_REASON: Bluetooth likes 120 lines

Change-Id: I1d043889016d48b43c0d455051d5c8151b48c2e8