Do not register + unregister DA in the same call

Calling unregister + register in the same call is NOT able to process
the onDisplayAreaVanished() until the register call is finished.

Depend on flag mIsOneHandedEnabled to handle register/unregister DA

Test: atest WMShellUnitTests
Test: atest SystemUITests
Test: manual quickly enable/disable OHM
Bug: 178357839
Bug: 179092435
Bug: 179985282
Change-Id: Iaafb1a0ab1ba9c16088ed91a420edf0421123e67