Implement PII Stripper, part 2

Implement EncodedBuffer that strip pii based on given privacy request.
The reason to implement another buffer is the length-delimited field's
size could change when its submessage gets stripped. It also intends to
keep the orignal data around for other requests to consume it.

In addition, the section implementation has adapted EncodedBuffer so
write out to each request's fd could be request-specific. The next step
is allow requests to set its privacy spec.

Notice the current design set the privacy spec of dropbox to AUTOMATIC,
this behavior might change in the future.

Bug: 64687253
Test: unit tests are writtern, see README.md for how to run unit tests.
Change-Id: I7ac236b8265ba9289dc6e17a8a5bf7f67ffb6bf5