Update javadoc for UiTranslation functions.

1. Provide more detailed information for UiTranslationManager.
2. Clarify the difference for VTC#onClearTranslation and
VTC#onHideTranslation.

Bug: 178044703
Test: n/a. Change javadoc not change code.

Change-Id: I4b2ef1a6c39c3bbbb85d0dcda519d286f10deb25