Remove double cache eviction logic.

WifiTracker and AccessPoint both maintain their own caches of
ScanResults and dictate their own grouping logic. This leads to various
issues, such as dropping APs during network selection. Remove the double
cache eviction logic, making WifiTracker the source of truth for
evicting and grouping ScanResults.

Consolidate on AccessPoint time based eviction logic. In place swap of
logic, with subsequent wifi tracker clean up and cache rekeying to
follow in later CLs. Logic was left as close to original location for
reviewer ease and will be rearranged in following CLs.

Moves existing key generation logic from AccessPointPreference into
AccessPoint.java

BUG: 64989100
Test: runtest runtest --path
frameworks/base/packages/SettingsLib/tests/integ/src/com/android/settingslib/wifi/AccessPointTest.java
runtest --path
frameworks/base/packages/SettingsLib/tests/integ/src/com/android/settingslib/wifi/WifiTrackerTest.java
make RunSettingsLibRoboTests
make RunSettingsRoboTests
Manual inspection of WifiSettings jitter when changing networks / moving
across floors.

Change-Id: Id30a34b08e14fc8da2c9d29dfcb5d1e8973cc18c