Increase size limit for pulled StatsEvent.

Increase StatsEvent max byte size to 50 KB when not using pooled buffer.

- 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 usePooledBuffer() is called, max size is set to ~4 KB but buffer
isn't resized. And if the current payload exceeds this limit, set
overflow bit to true.
- Only "recycle" Buffer object if its size is <= ~4 KB.

Bug: 158214941
Test: atest FrameworkStatsdTest
Change-Id: I9b6f73688b0bccb5a70f4ef51750464ec9c87fb9