Replace strtok_r() with C++-style android::base::Tokenize()

android::base::Tokenize() is like android::base::Split() but ignores
empty tokens. Think strtok_r() and strsep().
C++-ify parsing code by replacing strtok_r() with Tokenize(), which
results in more concise and readable code.

Bug: 204056804
Test: atest CtsFsMgrTestCases
Change-Id: Ic1dd0eb97ef9ce6786dc2bf65cdee22a40d813e7