Reland "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
(cherry picked from commit 3c1b581f)

Change-Id: Icce8c1b5ad074421052f68fa138d90adb85cca27