Squashed recents/rambar (2/5)

SystemUI: ram bar and recents performance improvements

PS2: add SettingsObserver to eliminate the need for SysUI restart
PS3: tab derp
PS4: stop jumpy behavior when clicking recents fast!
PS5: more reasonable timeout
PS7: restore :)

Change-Id: Iee304142285757656129ab92542013196615a404
Signed-off-by: Roman Birg <romanbirg@gmail.com>

SystemUI: simpler RAM bar implementation

Signed-off-by: Roman Birg <romanbirg@gmail.com>

Change-Id: I6b7c74c515b07269fa2ed9a96263fc47e58d5920