Correct JNI string error checking and miscellaneous corrections

Correct JNI string error checking and miscellaneous corrections

* src/android-emacs.c (main): Do not attempt to load the
bootstrap class path, which is redundant on all Android releases.

* src/android.c (initEmacs, android_browse_url): Do not assume
exceptions will be raised if GetStringUTFChars fails.  Decode
Android JNI strings as Qandroid_jni.

* src/androidvfs.c (android_saf_check_nonnull): New function.
(android_saf_new_mkdir): Likewise.

(cherry picked from commit 99e510977b22ca60e48b2af70a3c2cdbd90b2b01)