FileUtils: Do not expect Android root without an ending slash

Logic for constructing paths in IsLocationOnModule assumes that Android
root is provided with a starting slash and without one at the end. Even
though our default settings match that, some users have reported crashes
with a custom ANDROID_ROOT env variable. Change logic to support both.

Bug: 130817106
Test: set ANDROID_ROOT and compile
Merged-In: Id83ca985b05bd4e8f85d7ca0b4c8aeea98aa21ee
Change-Id: Id83ca985b05bd4e8f85d7ca0b4c8aeea98aa21ee
(cherry picked from commit 240cb2aa3c660f070e1fb1d630e01c13ba57c194)