Improve external storage CTS tests.

Change read test to no longer depend on existing file, and verify
that written values can be read back.  Remove testing of paths not
covered by CDD.

Verify that EXTERNAL_STORAGE environment variable is defined
correctly.  Improve multi-user storage test to validate legacy paths.

Bug: 7369656
Change-Id: Ic3ffc24e81297c739d2b07273957b01be2d05445
2 files changed