String8: fix infinite loop and segmentation fault in removeAll()

Bug: 290835996
Test: libutils_fuzz_string8 for several minutes

String8::removeAll() has 2 serious problems:

1. When `other` is an empty string, `removeAll()` will loop infinitely
   due to below process:

   a) with `other` being empty string `""`, find() will call strstr()
      on an empty string, which always returns `mString`, and thus
      find() always return 0 in this case
   b) with find() returns 0 for empty string, the next while loop in
      String8::removeAll() will keep loop infinitely as `index` will
      always be 0

   This CL fixes this problem by returning true if `other` is an empty
   string (i.e. `strlen(other) == 0`), this follows the logic that an
   empty string will always be found and no actual remove needs to be
   done.

2. When `other` is a NULL string, strstr() has undefined behavior. See
   https://www.open-std.org/jtc1/sc22/wg14/www/docs/n1570.pdf.

   This undefined behavior on Android unfortunately causes immediate
   segmentation fault as the current `strstr` implementation in bionic
   libc doesn't check `needle` being NULL, and an access to a NULL
   location is performed to check if the `needle` string is an empty
   string, and thus causes segmentation fault.

   This CL gives an error message and aborts instead of having a
   segfault, and to keep some backward compatibility.

   This CL also adds test for String8::removeAll()

Change-Id: Ie2ccee6767efe0fed476db4ec6072717198279e9