Add an option to maintain the focus mode when using touch-to-focus

There's at least one family of camera HALs that gets "confused" when
the focus mode forcefully changes to auto after setting focus /
exposure area options.

Sticking with the current focus mode still results in effective use
of the focus region, so don't force a switch to auto. The default
behavior remains the original, this needs to be explicitly enabled
on affected devices

Change-Id: Icdb1eb628f5ae4432bf03a64b38bbf054ba005a0