Limit MAP NewMessage notification to 7 days

Fixes a bug where "NewMessage" events were
incorrectly triggered for older messages.

According to the Message Access Profile(MAP),
a "NewMessage" event indiciates that a new message
has been received by the MSE device.

Since it is unusual to receive messages older than 7 days,
we can treat them false signals.

Bug: 310387636
Bug: 325863406
Test: atest BluetoothMapContentObserverTest
Change-Id: Ic0b1364be3f815ab5d30123614540c1e8245cfa9