Refactor MapClientContentTest to be form factor agnostic

Currently, MapClientContentTest is disabled if
PackageManager.FEATURE_TELEPHONY is not supported. Automotive targets
support the SMS/MMS database, but these tests are being ignored because
automotive targets do not support the Telephony feature.

Instead, MapClientContentTest should be form factor agnostic. This is
accomplished by replacing the real device's context (obtained via
InstrumentationRegistry) with a mock context instead.

Bug: 317905210
Test: atest MapClientContentTest
Flag: EXEMPT, test only
Change-Id: I03a0e676374f5dee05c89947e5eccdb3a3943e15