Remove tests checking other app's cache directory access.

Before R apps with WRITE_EXTERNAL_STORAGE permission had read/write
access to other app's cache directory. This behaviour has changed in
R and apps will not have access to other app's cache directories.
Removed tests from testExternalStorageWrite that assume legacy
behaviour.

Test: atest ExternalStorageHostTest#testExternalStorageWrite
Bug: 147091021
Bug: 148918640
Change-Id: Ic733da54b5dccdf8c9051844905bea13bc536b1e