gpuservice: Use startsWith() instead of compare() to check for a string prefix.

This fixes the behaviour in the case where the argument is lexicographically
greater than but not a prefix of "--gpumem=", as well as an out-of-bounds
read in the case where the argument is shorter than nine characters.
Found by running the CTS under HWASAN.

Bug: 139744799
Change-Id: Ie05bb31d9f490e45945ea26e1b03b9b5df4b976d