Write to external storage for DumpOnFailure

This is a partial revert of [1], to write the DumpOnFailure files again
in external storage. Reading them with the FilePullerLogCollector from
the app files only works if the device is rooted, which will not be the
case on CTS tests.

This will instead write them to the directory given by
getExternalStorageDirectory. While the test config has the path of
"/sdcard/..." for reading the files, this will get resolved to the
actual external storage path.

  [1]: I6dece3f39a2f3ee3273b6ef12e3314f807d3457f

Flag: EXEMPT testfix
Bug: 399050749
Bug: 395783236
Test: atest InputMethodServiceTest
Change-Id: I5d2e63427c46800d032972b6d74ff9a55377fae7