libcutils: fs_config: fix "system/<partition>/" aliasing

Pull prefix checking into fs_config_cmp and make prefix_cmp be a
glorified partial ? strncmp() : strcmp()

Results before:
[ RUN      ] fs_config.system_alias

[ ERROR ] system/core/libcutils/tests/fs_config.cpp:143:: vendor/lib/hw didn't match system/vendor/lib

[ ERROR ] system/core/libcutils/tests/fs_config.cpp:143:: system/vendor/bin/wifi didn't match vendor/bin/wifi

[ ERROR ] system/core/libcutils/tests/fs_config.cpp:143:: system/odm/bin/wifi didn't match odm/bin/wifi

[ ERROR ] system/core/libcutils/tests/fs_config.cpp:143:: system/oem/bin/wifi didn't match oem/bin/wifi
system/core/libcutils/tests/fs_config.cpp:247: Failure
Value of: check_fs_config_cmp(fs_config_cmp_tests)
  Actual: true
Expected: false
[  FAILED  ] fs_config.system_alias (6 ms)

Results after:
[ RUN      ] fs_config.system_alias
[       OK ] fs_config.system_alias (0 ms)

Bug: 62204623
Test: gTest libcutils-unit-test --gtest_filter=fs_config.*
Change-Id: I8282827002816e86ea3014fed29eabf43b837815
Signed-off-by: Ben Fennema <fennema@google.com>