Update UT cases for CarrierAppUtils

The CL includes the following changes:
- Introduce aconfig flag to protect the fix
- Add a new test case to cover the case when carrier app has
  been uninstalled before, it should not be uninstalled again.
- Add more space lines for each test case to improve readability
  (with given/when/then style).
- Update comment of test cases to make them more comprehensive

Bug: 158028151
Test: atest CarrierAppUtilsTest
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:a09e3f9506a312c07a04b90ca832142b693f52cc)
Merged-In: I30c5f65c8dac55d6a44862bc4a4ad5f256e17056
Change-Id: I30c5f65c8dac55d6a44862bc4a4ad5f256e17056