Fix SntpClient 2036 issue (1/2)

Fix issue with SntpClient after the end of NTP era 0 (2036).

This commit is 1/2. It makes some refactoring changes, lint fixes, adds
tests and introduces types that will be used in 2/2. Some of the added
tests fail and demonstrate the issue being fixed with the current
implementation.

-----

Failures that demonstrate the bug:

android.net.SntpClientTest#testRequestTime_era1ClientEra1Server

STACKTRACE:
junit.framework.AssertionFailedError: expected=5, actual=-4294967295995, allowedSlop=1
	at junit.framework.Assert.fail(Assert.java:50)
	at junit.framework.Assert.assertTrue(Assert.java:20)
	at android.net.SntpClientTest.assertNearlyEquals(SntpClientTest.java:502)
	at android.net.SntpClientTest.checkRequestTimeCalcs(SntpClientTest.java:215)
	at android.net.SntpClientTest.testRequestTime_era1ClientEra1Server(SntpClientTest.java:201)

android.net.SntpClientTest#testRequestTime_era0ClientEra1Server: FAILED (145ms)

STACKTRACE:
junit.framework.AssertionFailedError: expected=1139293696005, actual=-3155673599995, allowedSlop=1
	at junit.framework.Assert.fail(Assert.java:50)
	at junit.framework.Assert.assertTrue(Assert.java:20)
	at android.net.SntpClientTest.assertNearlyEquals(SntpClientTest.java:502)
	at android.net.SntpClientTest.checkRequestTimeCalcs(SntpClientTest.java:215)
	at android.net.SntpClientTest.testRequestTime_era0ClientEra1Server(SntpClientTest.java:174)

android.net.SntpClientTest#testNonMatchingOriginateTime: FAILED (116ms)

STACKTRACE:
junit.framework.AssertionFailedError
	at junit.framework.Assert.fail(Assert.java:48)
	at junit.framework.Assert.assertTrue(Assert.java:20)
	at junit.framework.Assert.assertFalse(Assert.java:34)
	at junit.framework.Assert.assertFalse(Assert.java:41)
	at android.net.SntpClientTest.testNonMatchingOriginateTime(SntpClientTest.java:384)

------

This commit:

+ Introduces a dedicated Timestamp64 type + test for holding NTP
timestamps.
+ Introduces a dedicated Duration64 type + test for holding the
32-bit signed difference between two Timestamp64 instances.
+ Fixes some naming to add clarity / addresses lint issues.
+ Adjusts tests

Tests are NOT expected to pass with just this commit. See 2/2.

Bug: 199481251
Test: atest core/tests/coretests/src/android/net/sntp/Timestamp64Test.java
Test: atest core/tests/coretests/src/android/net/sntp/Duration64Test.java
Test: atest core/tests/coretests/src/android/net/SntpClientTest.java
Merged-In: Ifdaada39298b05c48a3207fe6c0fad71c8a0a252
Change-Id: Ifdaada39298b05c48a3207fe6c0fad71c8a0a252