improve javadoc comments on CacheWordHandler

CacheWordHandler is the main interface, so it should be well documented.
rename CacheWordHandler.lock() to match CacheWordHandler.isLocked()

Clean up some unneeded redirection while we are at it.  If it is ever
needed in the future, it can be easily added back in.
rename INTENT_PASS_EXPIRED to INTENT_LOCK_CACHEWORD

This Intent causes CacheWord to lock, so this name makes more sense.
purge CacheWordSettings and directly set timeout in CacheWordHandler

This removes the now pointless CacheWordSettings and provides options for
directly setting the timeout in seconds.  This changes the meaning of the
non-positive values:

>0 = timeout in seconds
 0 = disable timeout
<0 = use default value

fixes #3718 https://dev.guardianproject.info/issues/3718
move getBlankServiceIntent() to CacheWordHandler to unify interface

This means that basically everything that the developer will ever need to
do can happen using CacheWordHandler.
replace duplicate code with call to getPasswordLockPendingIntent()