[adb data server] Initialize variable

servingComplete_ was left uninitialized and only set to 'true'
in the code.  We initialize it to the 'false' state to avoid
uninitialized references in SkipToRequest().

Bug: 150865433
Test: TreeHugger
Change-Id: Ia8a4d7135c432eb657543c5498fc9dbe8f4718b6