[adb] Optimize adbd's usb reading

Try to not allocate as many blocks on the heap, and reuse
memory instead of copying it

Get rid of unique_ptr and shared_ptr where possible, move
the Block objects themselves

Overall this reduces the time spent in memcpy() from 30% to
15% of the whole 'adb push' command, and gets rid of about 5%
of the time spent in the malloc/free calls

Test: builds
Change-Id: I8995115274b6f08a4df13c58183c928ef384a767