Implement IMM#showInputMethodAndSubtypeEnabler() in the client side

With this CL,

  InputMethodManager#showInputMethodAndSubtypeEnabler()

is fully implemented in the client side.  By not using system_server
privileged context to launch the Settings activity, we can stop
worrying about conflicts with existing security concepts such as

 * Background Activity launch blocking mechanism
 * Restrictions for instant apps

Bug: 34886274
Test: Manually verified as follows.
 1. Build aosp_coral-userdebug and flash it.
 2. adb install -r $ANDROID_TARGET_OUT_TESTCASES/ApiDemos/arm64/ApiDemos.apk
 3. adb shell am start -n com.example.android.apis/.ApiDemos
 4. View -> Input Method -> ShowInputMethodAndSubtypeEnabler
 5. Verify that the API works.
Change-Id: I5199ca4609c03900a0e86011d344b054b1df08dd