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: I341017e264ef9812e23bcb88da17b63da9ed03c6