Fix NPE in requestKeyboardShortcuts

Recents.getSystemServices() returns null if it has not created an
instance of Recents.

The CL replaces Recents.getSystemServices() with
SystemServicesProxy.getInstance(mContext) so that KeyboardShortcuts
class works independent of Recents.

Bug: 36873454
Test: Invoke Activity#requestShowKeyboardShortcuts
Change-Id: I3c2520695641607cc52f15506ba09ccdfe96940f
(cherry picked from commit 893348d3f66ba2a7f1ecd9eccaf34026923c77f3)