Revert^2 "Fix VDM IME handling:"

Differences from the original CL that broke ImeSmokeTest because
externally writing to secure settings to change the default IME
was broken: it reverted the setting stored for default device IME.

 - Instead of trying to keep the two settings entries in sync, simply
   reset the default device IME to null when current device BECOMES
   the default one. This way the default device IME setting should
   only be set while a virtual device IME is active. When an
   inconsistency is detected (e.g. system crash while on virtual
   device), then the setting is restored.

 - In resetSelectedInputMethodAndSubtypeLocked, reset the current
   device and display id. They'll be computed when the IME is about
   to show.

This reverts commit 455c2204.

Reason for revert: Issues fixed

Test: atest VirtualDeviceImeTest
Test: atest ImeSmokeTest with auto lunch target

Change-Id: I4f446f657197d0ad7d5cd32d9041a59fccc1453e