Increase size limit for pulled AStatsEvent

Increase AStatsEvent max byte size to 50 KB for pulled events.
All AStatsEvent instances are treated as pulled events unless
AStatsEvent_write() is called.

- Set the default max size to 50 KB
- The starting buffer size is still ~4 KB for pushed and pulled events.
- If a write would exceed the buffer bounds, double buffer size until
write fits or 50 KB limit is exceeded in which case the overflow bit is
set to true.
- If AStatsEvent_write() is called, max size is set to ~4 KB. And if
the current payload exceeds this limit, set overflow bit to true.

- Fix error mask checking in stats_event_test.
- Set ERROR_NO_ATOM_ID when atom id is missing.
- Make sure tests don't hit ERROR_TOO_MANY_FIELDS when testing buffer
overflow.
- Rename event->size to event->numBytesWritten

Fixes: 158214941
Test: libstatssocket_test
Change-Id: Ia26aeb775f7e4f2ffe87707bab6d0119e21da10e