Avoid const value_type of vector

This change is required to build fluoride against libstdc++.
libstdc++ asserts that: std::vector must have a non-const, non-volatile
value_type.

Bug: 176846220
Tag: #floss
Test: atest --host bluetooth_test_gd
Change-Id: Iabf796dc356826782b480fd01887c04a86e9708a