Fix KeyPairGeneratorTest#testGenerate_RSA_ModernSpec_AsCustomAsPossible

The test got broken on some devices by
4953e08d240acce21daf94caf8a809ce6804ed56 which added code to remove an
element from a read-only List. This CL fixes the issue by switching
the code in question to operating on a mutable list.

Bug: 23295907
Change-Id: I7a00ec9aa7bb863535301a63abb7b2b95865afe8
1 file changed