Preventing deleting file that does not belong to parts directory.

b/414388731#comment18. I'm adding this change to err on the side of caution, but it may be redundant given the recent sql injection protection changes.

bug: 414388731
Flag: EXEMPT bugfix
test: new test + existing coverage from testMmsPartDelete_canDeleteById.
(cherry picked from commit fce8998a1770d05a768c30e5822c25db1b758208)
Cherrypick-From: https://googleplex-android-review.googlesource.com/q/commit:9ce5c9d9bd63c7db1ba5ff3e3e834e03b00d0fff
Merged-In: Ic2072a12bf93be7c17a86b372ae7e7057d7eb237
Change-Id: Ic2072a12bf93be7c17a86b372ae7e7057d7eb237