Allow TTS service to identify caller.

Added new field (+setter and getter) to SynthesisRequest with
Uid of a calling process. TTS service will be able to discover
packages names associated with caller using
PackageManager.getPackagesForUid.

This will allow to block buggy or poorly designed programs from
an unintentional DDoS attacks against TTS service.

Bug: 8625440
Change-Id: I5ac0ea191f952495c00301f17efdf28205353ae4