Support multiple key handlers

Convert the string overlay to a string-array overlay
to allow devices to specify an array of key handlers.

Note that the keyhandlers towards the start of the
array take precedence when loading.

*DU edits*
This is 95% this commit and 5% other stuff, but all from the same author.
So to end my headache, here is 1 more commit to add to Device-Key_Handler.

Change-Id: I5a38129751ead0eed85816a4bb9ac2159ff6d134
Signed-off-by: Mohammad Hasan Keramat J <ikeramat80@gmail.com>