Prefer mocking TelephonyManager in PowerButtonEndsCallPreferenceControllerTest

The PowerButtonEndsCallPreferenceControllerTest is the only one
relying on ShadowUtils to mock the isVoiceCapable() logic. Update it
to use a TelephonyManager mock to be in line with other unit tests.

Bug: 395714454
Test: mm && atest PowerButtonEndsCallPreferenceControllerTest
Flag: TEST_ONLY
Change-Id: I6e19fea142a9aaf12ac28476fc5a97c82022d4f4