Move canBeHiddenByKeyguardLw and correct its condition for activity

- Fix an accidental behavior change by commit cb239e07.
  Originally, all windows belonging to an activity should not be
  always hidden by keyguard, e.g. starting window.
- Reduce unnecessary abstract invocation.

Bug: 207587841
Bug: 163976519
Test: CtsActivityManagerDeviceTestCases
Change-Id: I1a6f38535800e40669e8c31a0aa65477e25b1d46