DropBoxManager: use BufferedInputStream when using GZIPInputStream

GZIPInputStream has a bug (b/195554062) where it uses unbuffered IO
when reading the gzip header.  This triggers the UnbufferedIoViolation
strict mode violation for apps that target API level 28+.

Independently of fixing GZIPInputStream, DropBoxManager can use
BufferedInputStream to take a proactive step against this and future
bugs.

Bug: 195554712
Test: Successfully read gzip dropbox entries from API level 28+ app.
Change-Id: Id539efcb366b3e3d53513a098975b25866ed16ff