Add instrumentation flag to get access to full exteranl storage.

When starting a process under instrumentation using this new
"--no-isolated-storage" flag, the process would be able to see the
full external storage.

Bug: 117229024
Test: adb shell am instrument --no-isolated-storage \
      -e class android.media.cts.EncodeDecodeTest#testEncodeDecodeVideoFromBufferToBuffer720p \
      -w android.media.cts/android.test.InstrumentationTestRunner
Change-Id: I7973b123cf4fc08e8ce2b05bd4c23fa41b1cdcdf