Improve SystemUIBootiming_StartServices_avg

Previously, we tried to tuning boottime of SysUI
See go/systemuiboottiming_ohm
- Reduce access setting provide at SysUI start-up
- Remove unused obervers
  1. Timeout
  2. TapAppExit
- Update mShortcutEnabled state at ctor

Test: manual enable OHM shortcut, rebot
      make sure shortcut function works
Test: atest WMShellUnitTests
Test: asit/perf/boottime_test
Bug: 193225130
Change-Id: I2753f8b1e47622c00b9c6d2f4cd319c6e43634ea