Ignore double tap events when proximity sensor is covered

Differentiate between power key events originating from actual button
and ones originating from the touch screen. The latter must be ignored
when proximity is covered.

[mce] Ignore double tap events when proximity sensor is covered
Plugins remove sensor input callbacks when unloaded

If sensor input callbacks in plugins are left in place when plugin
is unloaded, terminating sensor framework sessions on mce exit path
can end up with segfault.

[mce] Plugins remove sensor input callbacks when unloaded
Merge pull request #93 from spiiroin/double_tap_device

Ignore double tap events when proximity sensor is covered