Only clear selection if select size larger than 0

Many crash bug is occured while clearSelection onDestroyActionMode.
By log pattern, selection change to 0 so it call action mode finish.
In this case, selection size is 0 so we should not call clearSelection
again.

Fix: 143999752
Fix: 144862372
Test: atest DocumentsUIGoogleTests
Change-Id: Ib50912e00dce62535e455dee431e142021ec7cba