Screen capture: Fix region box tap target size and detection logic

Previous to this fix, the tap target size was actually double of what it
should have been due to erroneously adding the total size in both
directions instead of half in each direction. Also renames the method
and parameters to more clearly indicate meaning and intention.

Bug: 435503791
Test: Manual
Flag: com.android.systemui.large_screen_screencapture
Change-Id: I531575e8a9504a25014d5f2902f11560ca7d27be