Allow starting Emacs --debug-init on Android

Allow starting Emacs --debug-init on Android

* doc/emacs/android.texi (Android Troubleshooting): Document
`debug-init' option.
* java/AndroidManifest.xml.in
(EmacsLauncherPreferencesActivity): New activity.  Export on
systems older than Android 7.0.
* java/org/gnu/emacs/EmacsActivity.java (onCreate): Adjust for
string startup argument.
* java/org/gnu/emacs/EmacsLauncherPreferencesActivity.java: New
file.
* java/org/gnu/emacs/EmacsPreferencesActivity.java
(EmacsPreferencesActivity): Don't make final.
(startEmacsQ): Give start-up argument as an argument, not as a
boolean.
(startEmacsDebugInit): New function.
(onCreate): Register new listener; make final.
* java/org/gnu/emacs/EmacsService.java (onCreate): Pass
extraStartupArgument.
* java/org/gnu/emacs/EmacsThread.java (EmacsThread): Rename
startDashQ to extraStartupArgument.
(run): Adjust accordingly.
* java/res/values-v24/bool.xml:
* java/res/values/bool.xml:
* java/res/values/strings.xml: New files.
* java/res/xml/preferences.xml: Add new option.  Move string
resources around.