Fix getSwitchState.

InputReader::getSwitchState always returns AKEY_STATE_UNKNOWN
because SwitchInputMapper::getSources() returns 0 which cannot
match any source mask including AINPUT_SOURCE_ANY.  As a result
initial lid switch detection is broken.

This change adds a new source constant AINPUT_SOURCE_SWITCH
that indicates that the source has switches.

Change-Id: I5321ecf0ce84f1c2b4535f6c163d3f4dcf9b7a9b