Do not compare mediaId, genre, duration in Metadata.equals()

The commit "b4d77324: Make Metadata.equals() faster" added more fields
into Metadata.equals(), but in turned out that they are removed
intentionally. This CL excludes the fields comparisons from the method.

Previous commits to refer:
"97229a2: Add the MediaPlayerList and fix some checks"
"de1bb34: Keep track number in sync when playlist has..."
"b4d77324: Make Metadata.equals() faster"

Bug: 265604734
Test: atest GPMWrapperTest
Change-Id: Ieec018bde9de2a5508ae27cbd417e0d5a33a08aa