Change StorageHostTest#testFullDisk to case-insensitive match.

I'm not sure why this test ever passed on Pixel - the dialogs shows
'Delete', not 'DELETE' (and as far as I can tell, that hasn't changed
recently). Change to a case-insensitive match so we allow both.

Bug: 247632239
Test: atest StorageHostTest#testFullDisk
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:e641b1ebfb5e43928a1c88d7a09e09385df65775)
Merged-In: I0a29078e6873e932d1a9f8beac3de38e35d20a86
Change-Id: I0a29078e6873e932d1a9f8beac3de38e35d20a86