aconfig: reject consecutive underscores in identifiers

The Java codegen may translate flag names (snake_case) to Java
camelCase, dropping the underscores. The flags a_b and a__b will
translate to the same camelCase form, which is ambiguous.

Circumvent this problem by disallowing consecutive underscores in flag
names, flag namespaces, and packages.

Bug: 284252015
Test: atest aconfig.test
Change-Id: I2586a38160723c06265a140193da8178655553e4