Removes camera service prewarming from bottom area.

Back when we supported swiping sideways to show the camera preview, the
KeyguardBottomAreaView had functionality for prewarming the camera
service. Presumably, this was designed to make sure that the camera
preview is connected as soon as possible.

These days, we no longer support the sideways swipe to show the camera
preview. Instead, we rely on the double power button tap gesture to open
the camera.

Therefore, the functionality in question is no longer needed and can be
removed.

The greater context for this change is the desire to refactor/modernize
the KeyguardBottomAreaView. The smaller the API surface this class
supports, the easier the refactor will be.

Test: Manually verified that the lock screen still seems to behave the
same way as it did before this change. Also locally ran the
KeyguardBottomAreaTest test. Also, swiping sideways on the lock screen
does not crash or anything like that.
Bug: b/235403546

Change-Id: Ia369d01332d3ae3cd757ff86c15b06a9e495d703