Clear request list while timeout.

The wakelock will be kept held if there is outstanding requests
in request list. When WAKE_LOCK_TIMEOUT occurs, all requests
in mRequestList already waited at least DEFAULT_WAKE_LOCK_TIMEOUT
but no response. Those lost requests return GENERIC_FAILURE and
request list is cleared.

bug:3292426
Change-Id: I369c6ba4d6836d65ef616140e48c7304faf888f0