Add condition to useSurfaceBoundsAsTouchRegion.

This CL adds a condition so that useSurfaceBoundsAsTouchRegion is only
set to true when a client did not provide a touchable region already.
Otherwise, the touchable region provided by the client will be replaced
by the surface bounds.

Test: atest SetTouchableRegionTest SurfaceFlinger_test on freeform
windowing emulator and aosp_cf_x86_64_phone target
Bug: 272786078

Change-Id: I2b296bc3f3d632b6ddbf2438b2aae681436ea281