Bug 947918: Support seeking to end of BlobView buffer

Reading or advancing through all bytes in a BlobView buffer sets
the index to the length of the buffer. This patch changes 'seek'
to work the same way, so it's possible to seek to the buffer's
end.

Signed-off-by: Thomas Zimmermann <tdz@users.sourceforge.net>
Merge pull request #14502 from tdz/bug-947918

Bug 947918: Support seeking to end of BlobView buffer r=padenot
Bug 890434 - Impossible to silence an incoming call without rejecting it

We use Inter App Communication to make this work
- System app sends a message in 'dialercomms' when we press the power or volumedown buttons
- The Call Screen stops ringing and vibrating when receiving this message

I moved the Audio mock to shared because it is now used by 3 apps.
Had to modify some sms tests to work with the new shared mock.
Merge pull request #14577 from Rik/stop-ringtone-power-button-890434

Bug 890434 - Impossible to silence an incoming call without rejecting it r=etienne
Bug 933203 - [DSDS] Operator variant logic: APN settings