Remove lock for internal isGetTasksAllowed

The method only checks recents and permission. The former is a field
which won't change since boot. The latter doesn't involve any WM
stuffs. Other places also use it without lock.

This avoids unnecessary lock contention when calling ActivityManager#
getRunningAppProcesses, getProcessMemoryInfo, getRunningServices.

Bug: 204870457
Test: Test: CtsWindowManagerDeviceTestCases
Change-Id: Ie1f1d1f58543cf25197f42caa01565039ced4b32