Make synthesizeToFile create file on a client side.

In previous setup, synthesizeToFile method relied on synthesizer
service to create world readable output file. This is potential
source of vulnerabilities.

This change moves output file creation to the client side, and
synthesizer service receives already opened file descriptor.

This change may break applications that are creating files in
now unaccessible locations, like /sdcard/.

Bug: 8027957
Change-Id: I97351be5d2f2f8ef9aa43d0ab08c4b825ca4c22b