Enable per-profile IME by default

With this CL, per-profile IME will be enabled by default.

Note that on debuggable builds (Build.IS_DEBUGGABLE == true),
developers can still disable per-profile IME mode as follows.

  adb root
  adb shell setprop persist.debug.per_profile_ime 0
  adb reboot

Note that multi-client IME [1] is a completely different story,
because it was designed to be fully multi-user / multi-profile aware
since its begging.  Enabling multi-client mode means that per-profile
mode is always enabled, because that's the only mode that
MultiClientInputMethodManagerService supports.

 [1]: I41dfe854557b178d8af740bc2869c936fc88608b
      bae5bea2

Fix: 63907246
Fix: 111083076
Test: atest CtsAdminTestCases CtsDevicePolicyManagerTestCases
Test: atest CtsInputMethodTestCases CtsInputMethodServiceHostTestCases
Test: Did multiple manual tests.
  * OTA scenario from older builds
  * Unlocking devices by typing password
  * App focus changes between personal apps and work apps
  * Direct-reply notifications from personal / work users
  * IME whitelist is only enforced to work IME settings
Change-Id: I854ce92b2bf3aab49f14f6cde444acf2182b9ad0