Enforce READ_DROPBOX_DATA permission for ecall diagnostics.

Following privacy review done for the recent mainline changes to resolve
TelephonyManager#persistEmergencyCallDiagnosticData, ensure that
READ_DROPBOX_DATA is explicitly required. Since the new changes have
been gated by a flag, this permission check needs to be conditionally
checked on the same premise.

Bug: 328132903
Test: Manual
Test: atest TelephonyManagerTest
Change-Id: I67e08a9e8279ab44593ed07d0fa44fdebb953899