Support non-default back-disposition in ThemedNavBarKeyboard

With this CL, you can let ThemedNavBarKeyboard call

  InputMethodService#setBackDisposition(int)

with either the following values

  * BACK_DISPOSITION_DEFAULT
  * BACK_DISPOSITION_ADJUST_NOTHING

so that you can see how those two flags work.

Bug: 213337792
Test: Manually verified as follows
 1. Flash aosp_coral-userdebug into the device
 2. make -j ThemedNavBarKeyboard
 3. adb install -r $ANDROID_PRODUCT_OUT/system/app/ThemedNavBarKeyboard/ThemedNavBarKeyboard.apk
 4. adb shell ime enable com.example.android.themednavbarkeyboard/.ThemedNavBarKeyboard
 5. adb shell ime set com.example.android.themednavbarkeyboard/.ThemedNavBarKeyboard
 6. Tap any edit field
 7. Make sure that the back button is left-pointing triangle
 8. Tap "BACK_DISPOSITION_ADJUST_NOTHING" on the IME
 9. Make sure that the back button is down-pointing triangle
Change-Id: I71aa1dd23d72e8891cb3887efe5db1a48b6a017d
1 file changed