Desktop screen capture: set correct button text for screen recording

This CL changes the text of the button in the region box to say "Record
selected area" instead of "Capture selected area" when screen recording
is enabled.

Bug: 436891244
Test: Manual
Flag: com.android.systemui.large_screen_screencapture
Change-Id: I0c1de85583e8e5c52c5f401f77068c6576dba1f5