aconfig: change java flag method name to camelCase

Before java code will directly use the flag name as the method name.
This change adds funciton to try the best to convert flag name to
camelCase, and then use the camelCase string as the method name in the
generated code.

Bug: 279483816
Test: atest aconfig.test aconfig.test.java
Change-Id: I45fc6df46c9d535cd38a657a41313202f9b660af