Check & request operation missing before using RingtoneManager.getCursor()

[aper-project]

Issue description

Hi, in ClockPlus v1.1.3, we found a dangerous API usage (https://github.com/philliphsu/ClockPlus/blob/master/app/src/main/java/com/philliphsu/clock2/dialogs/RingtonePickerDialog.java#L85) which requires Manifest.permission.READ_EXTERNAL_STORAGE in accordance to the Android official documentation (https://developer.android.google.cn/reference/android/media/RingtoneManager?hl=en#getCursor()).

However, it seems that it missed the “check” and “request” operation in the following call chain starting from the BaseAlertDialogFragment.onCreateDialog(android.os.Bundle) activity if permission is not granted.

CALLCHAIN:
	com.philliphsu.clock2.dialogs.BaseAlertDialogFragment.onCreateDialog(android.os.Bundle)android.app.Dialog
	 com.philliphsu.clock2.dialogs.RingtonePickerDialog.createFrom(android.support.v7.app.AlertDialog$Builder)android.support.v7.app.AlertDialog
	  android.media.RingtoneManager.getCursor()android.database.Cursor

This may lead to a SecurityException or related functions unavailable if the user denies the access permission but still calls the API in this chain, resulting in bad user experience.

@philliphsu
Could you help me review this issue? Thx