Make Mtp FFS allocations per session rather than per file

Workloads that send a large number of small files could
repeatedly allocate and free the buffer, causing fragmentation
and eventually running out of allocable memory. Instead
have the allocation be once per MTP session, and retry
with smaller buffers if it fails initially.

Bug: 34741015
Bug: 34822471
Test: Transfer files via MTP
Change-Id: I775376076d3a0c26765b211100830ea0c08450ef