Removed unnecessary global lock for register/unregister capture observer method.

The registerScreenCaptureObserver and unregisterScreenCaptureObserver method have added the wms global lock,we do not need add lock in this two methods.

Bug: none
Flag: EXEMPT minor optimization
(cherry picked from https://android-review.googlesource.com/q/commit:a49dd8ea7fa41e2dc02c2dcaa28d63774d45994c)
Merged-In: I5db6342fa1a34f225200296d54539c36501b8d73
Change-Id: I5db6342fa1a34f225200296d54539c36501b8d73