We currently use intent filters to direct jackpal.androidterm.RUN_SCRIPT
actions to the RunScript activity, which is gated in the manifest by the
permission jackpal.androidterm.permission.RUN_SCRIPT.  However,
RunScript is an alias for the RemoteInterface activity, which is exposed
without the permission requirement; because applications are permitted to
explicitly specify a component when sending an intent, this allows an
application to trigger a RUN_SCRIPT action without having the
corresponding permission.

Fix this by removing RUN_SCRIPT handling from RemoteInterface and
creating a new RunScript activity (a subclass of RemoteInterface) to
handle RUN_SCRIPT actions.  Any scripts supplied by callers to
RemoteInterface will now be ignored, though the call will open a new
terminal window.

Thanks to Daoyuan Wu for reporting and analyzing the issue.  Fixes #374.

The main Term activity implements a set of private intents for use by
the remote interfaces.  These are only supposed to be used via the
TermInternal activity alias, which is private to TEA, but it's possible
for another application to invoke these on the main Term activity by
explicitly specifying a component name (as in #374).  It should not be
possible to do more than crash TEA via these intents, but let's prevent
outside applications from using them anyway by ignoring requests which
don't come via the TermInternal alias.