Remove usage of base/string/* in libfs_avb

As part of the effort to uprev libchrome in Android, we are
removing Android dependencies to libchrome utility functions
that are not strictly required.

This CL remove the usage of base/string/* and replace by
android-base/string*

Bug: 360917504
Change-Id: I89e7bdd66bdf8f565f6b2084a70335616a644069
Test: atest libfs_avb_internal_test
5 files changed